Maybe it's because drinking some Soylent isn't an 'activity' like eating a prepared meal is, but rather just something I do while doing something else, so I don't notice it as much (which I think is part of why it's designed to be neutral tasting).
But even putting your feelings aside, the issue with the same meal over and over is that it does not provide your body with what it needs. Soylent solves that: have Soylent and you don't have to worry about varying your meals, or about how much salt, sugar or fat you are eating.
And going back to the feelings side, Soylent feels different. It's supposed to be like that, and that somehow makes it feel much more okay. There are also variants that add flavors (making me worry about sugars again, but not as much as with normal food), for example I usually order a week's worth of food which comes in 5 different flavors (so you have two duplicates). I take about a year to consume them though, it's not like I run on Soylent full-time.
I eat exactly the same breakfast and lunch 5-7 days a week. It's easy, relatively cheap, and the routine keeps me from screwing up my calorie counts.
Dinner has some more variety, but I'd be up for pizza every day if it weren't for the cost and health issues.
Not a totally fair comparison here, since our neighborhood pizza places routinely show up on "best pizza in the US" lists...