I suspect the reason is consistency. For the most part, getting something at a large chain is going to be pretty similar over time regardless of the location, so if you're really in the mood for something specific, it can be easier to go with a known quantity even if the overall quality is very mid. My wife and I have a bit of an in-joke every time we try a new restaurant about getting French fries because I tend to strongly prefer fries that are crispy, whereas she enjoys fries that are more "soft" as long as they're not soggy (like steak fries, crinkle fries, etc.) office times seeing fries on a menu doesn't make it at all clear what exactly you're getting, so probably at least half the time I end up just giving her fries that I order because they end up not being what I'm looking for. Getting fries from McDonald's or Wendy's or something isn't going to end up with you having the best fries you've ever tasted, but you know exactly what you're getting at least compared to a small individual restaurant, and for those of us with very specific preferences, it's just easier sometimes.

Even though this is a bit tangential (and not something you're calling out here), I'll also add that having struggled with being picky with things that are ostensibly the type of foods that picky eaters enjoy like fries has led to me to absolutely loath having to ask for details about stuff in restaurants. It probably doesn't help that I'm also on the spectrum and have suffered from social anxiety a lot over the years, but I'll never understand why so many people feel so entitled to demand explanations about why I don't like certain foods or ways that they're prepared when it doesn't affect them at all. It's not like I _want_ so many foods to taste worse to me than others, and yes I'm sure that with the right amount of effort and diligence I probably would eventually be able to get past most of it, but I don't see why that means that acquaintances or coworkers I'm not particularly close would think it's reasonable to interrogate me about it; if anything, having to try to explain myself makes it _more_ stressful and reduces the emotional energy I have to try to change things! Back when I worked in an office, I sometimes would literally try to time my lunch break to when nobody else would be in the cafeteria because so many people over the years would try to get me to explain why I would eat get food from the same restaurant multiple days in a row, and I got to the point where I would have anxiety about eating in front of other people. This isn't even touching on stuff like how hard it seems to be for some restaurants to follow what I'd consider to be very basic instructions (like "don't put mayonnaise on this sandwich" and how often times the "default" way something will be served will contain extra condiments or toppings that weren't even listed on the menu, and trying to deal with that just adds to the stress given how often times asking for the issue to be just leads to frustration on both sides. My point is that sometimes the choice of what to eat isn't just a choice of how to ingest calories but has social implications as well, and I think that attitudes like "I don't understand why people do this" can end up just reinforcing trends where some people will value consistency. If I'm hungry and just want to eat and get back to whatever I was doing beforehand, sometimes I'm just going to want to know exactly what I'm gonna be dealing with in advance, and unless I happen to already have spent the time to try out enough places to know what I like in the area, a national chain is a way for me to be reasonably confident about that.