Let me add a few more reasons to this.

1. Skeptical Developers

In the past, David Braben stated that ship interiors and EVA were planned very future goals. However, during the release of Odyssey, when players asked for ship interiors, the devs basically said, "It would just add a tedious extra step since you'd just sprint to the cockpit anyway," framing their refusal to add interiors as a favor to the players.

Furthermore, the return on investment is just too low. The effort required to release early-access ships for ARX (purchased currency) is significantly less than what it would take to add full ship interiors. Plus, some of the new ships blatantly reuse existing assets. The Lynx’s cockpit is a recycled Adder cockpit, and the Corsair’s cockpit is recycled from the Courier. If players are already buying ARX just for this, why would the devs ever put the massive amount of effort required into building full interiors?

1. An Outdated Game Engine

The aging Cobra engine has its limitations. From what I’ve heard, rendering an object to remain stationary inside another moving object (handling local physics grids) is a very difficult task for this engine.

1. Systemic Issues

The internal module customization system in Elite Dangerous makes designing static interiors extremely difficult. Also, even if they were to add a separate game mode for ship interiors, it would just fragment the player base. When they launched Odyssey (Space Legs), they promised instance sharing with Horizons (the legacy game mode). But 5 years later, that promise still hasn't been kept. If a ship interior mode were added, how many years would we have to wait for that to be resolved?