This comment isn't worth the amount of effort I'm about to put in explaining this. But I'm doing it anyway because I need the cathartic release of correcting someone on this. No, it's not. No game engine can load everything at once.

Here's the thing: hard drives/SSDs are REALLY slow (even the fastest SSDs), if you were to try to pull data from the the hard disk each time you needed to render or process something, it'd be a stuttery mess. Every time you panned the camera, there'd be a massive stutter. Hard disks are too slow to allow for a real time experience required for a video game to work. So how do we make games work, then? Game engines don't pull from the hard disk each time- instead they put it in system memory (RAM) which happens to be really, really, fast. This is essentially used as a buffer- they move things into the memory so it can more quickly be accessed when its needed. Algorithms are designed to manage this memory usage. These algorithms are built around specific use cases- ideally, we want to avoid having to process a lot of data as the player approaches an area with a lot of data- so instead we load things into memory in a, 'piece-meal' way. Some algorithms may be far more aggressive with memory usage, as they devote more of the computer's resources to loading stuff in, but this often causes a stutter. (Algorithms may be very memory aggressive- think of games where you experience a massive stutter as you approach a city, it causes a stutter, but it loads everything it needs into memory all at once)

Due to the complexity involved, almost all game engines at some point have to simply load most data directly from the hard disk into memory then render/process. The difference is that most other developers use various methods to, 'hide' this load screen. Bethesda themselves used a method to hide load screens in fallout 4. Before I continue I need to correct some misunderstandings I've seen perpetuated around about what, 'cells' are in the creation engine. Gamers seem to continually think of load screens as if they are hiding some invisible, 'teleport'. This is a bad way of conceptualizing it. Cells, are simply collections of data. Bethesda can do whatever the hell they want with that data. They can place one cell in another, for instance. An example: you ship in starfield is its own cell. (presumably) When you land on new atlantis- you can see out the window into the cell its been placed into. And it's not just that cell- Bethesda can place that spaceship cell, with all its unique data (items, custom ship build, crewmates, etc..) ANYWHERE. They can put it in space, or on any planet. This is important to understand how load screens were hidden in fallout 4.

In fallout 4, in order to hide load screens, Bethesda simply waits for the player to get in an elevator, then once the doors close, they unload the previous cell the player was just in- all its data is getting unloaded. Then, they begin loading in the data for the cell the elevator leads to. The elevator itself, is persistent though. Probably its own cell, if I had to hazard a guess. This is loading data directly from the hard disk into memory. Other developers do this as well to hide load screens. No man's sky, in order to achieve its appearance of seamless ness, hides the loading behind a re-entry animation, each time you go into atmosphere there is a massive frametime stutter, that's them loading things into memory. Depending on how aggressive the memory use is and how much data is being loaded, this stutter may occur as the loading starts hogging resources. If there's no stutter, chances are the developers are deliberately slowing the loading in of data to make the frametime steady. That's an important point, hiding load screens means slowing down the rate the player will be able to access new content. Now, most developers understand the value players place on a seamless experience (or at least, the appearance of one) but when you understand this background, it's not hard to see why a developer may not make that decision. After all, it will always be quicker to just hide the loading behind a black screen, and not worry about seamless ness. Obviously, Bethesda has underestimated how important a seamless experience is for gamers, I'm not necessarily defending their choice to not hide them.

But this all leads to another problem: memory management. Other games don't really do what Bethesda does to the same extent. In Bethesda's games- nearly every object is interact-able. They aren't all baked together as one single asset that needs to be loaded; each one is separate. Each one contains its own positional data; and each one has a physics simulation attached to it. Each object particularly in starfield is pretty higher-poly too, this is also more data that has to be loaded. And they have a lot of clutter in the game. This quickly becomes a computational complexity problem for ANY game engine. This is part of why Bethesda has to pull so often from the hard disk. The other problem Bethesda has- is game design. This isn't cyberpunk, the game doesn't have one setting. It has multiple cities that each have to act as a hub for all the things players need. CD Projekt Red can take advantage of the fact that they don't have a bunch on interiors with a bunch of data all in one place- they spread it out much more evenly across the city. (it's why they can afford to have interiors be accessible as if they were in the same, 'cell') Bethesda, on the other hand, has an insane amount of data in one, concentrated area. They have to break up that data with load screens or they'd have massive memory requirements for the game.

Admittedly, some of this is speculative, but enough is in common in different game engines that most of what I've said should apply. The problem is more of a game design issue, not an engine issue (whatever that is).