And then something like The Expanse comes out and it turns out that realism is actually really interesting. Sure, the space is unfamiliar realism, but so is serf life to most viewers. And direction is also very important.
Anyway, you mention Space Marines, there's animations and lots of media about them. Some depicting them as basically invulnerable (like the 40K episode of Amazon's Secret Level), but plenty of them where they die en masse - because while they're super suits, they're up against the worst the universe can throw at them (like the British).
This comes from it fundamentally being small scale skirmish game. So realistic army sizes are not possible. And on other hand you need some level of game balance. You can't expect one side to have dozen models and other to field thousands or tens of thousands.
And even there. Considering stated population of any reasonably build world to be in billions and more populated to go to hundreds of billions. Number of normal humans you could stick a weapon in hands and told to shoot at that direction would still be in at least millions if not billions. A few thousand whatever can do very little against that.
Maybe it was Interstellar.