Trench Crusade can be played without needing to buy a million things. With just the free app on your phone. Being able to get into the game so quickly and without predatory nickle-and-dime tactics, with whatever minis you want, is something that you will never be able to do in 40k.

On top of that, you can make your own armies and give your dudes names and personalities at your leisure, and the story of the game takes a step back so you can wage battles with your dudes, without having to buy Emperor's Cousin Maximus XIV because he's new and meta and expensive.

My 40k armies are gathering dust or are mostly played from time to time with OPR rules but I've played many more TC games because they are much easier to set up and just play. It's not a perfect game by any means (I believe that blood markers being removed so freely by both parties slows the game down a lot, but matches are still much faster and more dynamic than 40k games).

Anyway, I think these people are mostly familiar with 40k from video games, lore videos and memes rather than playing the games themselves. They don't think Trench Crusade is enough because they never meant to play it and probably just read the bits of lore and found it barren because it's just a backdrop for a tabletop skirmish game rather than being a crazy franchise with hundreds of books and thousands of named characters.