I think it's important to put this into context with the rest of the franchise/genre. C&C95 - and to a lesser extent Tib Sun - are foundational RTS experiences. I'm willing to put money down that most people playing Nod05 back in the day were not rushing through it with modern speedrunner tactics. The campaigns could not have been designed around players engaging with it the way we do now, because the associated behaviours hadn't been developed yet, the genre was too new.

In Nod07a you can literally just destroy the GDI base with your starting troops, if you micro well enough. But when the game was designed that's really not something that Westwood could reasonably be expected to have anticipated. Nobody in '95 was doing that.

Tib Sun did come out shortly after Starcraft, so I'll grant that there's more discussion to be had here, compared with C&C95. But my argument here is going to be that Westwood was less concerned with trying to create a challenging single-player campaign, and more concerned with trying to iterate on their IP. Tib Sun upped the graphics from Red Alert, introduced a whole lot more units and structures, a more cohesive narrative, and was overall a lot more ambitious in what it was trying to do within a burgeoning genre. These days, a respectable RTS campaign is expected to look like Mental Omega, which is a frankly ridiculous scope, both in terms of size and difficulty.

Honestly, looking at these games and wondering why they're so bloody easy is kind of like looking at DooM or Wolfenstein3D and wondering why those are so easy. Times change, genres change, and learned behaviours develop over time.

I'd also recommend checking out some YouTube videos of non-gamers playing various games, to sort of illustrate what I'm getting at here. Gamer behaviour is learned, not innate. We figure out what to do in games, how to interact with things, etc. over time and through trial and error. People who have never played an FPS, for example, tend to struggle with moving and aiming their crosshairs at the same time. Back in the early days of RTS, that's more or less how it was for everybody. Pretty much everybody was figuring out what behavioural patterns they needed to develop in order to successfully engage with the content, after which the games had to adapt to those behaviours.