I feel like I've explained the meaning of "expensive" already in detail in my comment. For this specific example, yes, you're right, but i'm not eating only lentils every day of my life. I need a balanced diet of different things, some easier to get/cook than others. I also need to figure out what I like and try things and replacements for things I already like, maybe that's just me but habits are very hard for me to change and require an enormous amount of mental energy. Every little habit is something I don't have to think about or spend energy on, it's automated : I know what it is, where to get it, how to cook it, I like it, etc..

The expensive vege/vegan things are pre-prepared food and such things where you essentially buy the time/energy you don't have with more varied/nicer things, and I do buy some of those because I don't have a lot of time/energy. I'm trying to buy less of them and cook them more myself, but yet again that's a lot of time/energy.

Edit: also something else I thought of, vegetarian is now way easier and eating lentils to me is more vegetarian. Vegan is a bit of a step further in the amount of research and work you need to do to make sure animals aren't involved in the process, because holy shit the amount of times i've found out something I thought was innocuous is actually not vegan-friendly.