Let's take some time to explore, shall we?
Allowing for the pretty intense personal stuff you have to go through in order to be in a poly relationship, the actual relationship, in my opinion, is actually way easier.
Reason 1: "The One" Mindset
I hate this comic a lot, but I'm going to post it just for sake of argument.
The description of the comic says it's from Plato's Symposium. Which, if you actually read the part of the Symposium that talks about Zeus splitting humans into two halves, it's a lot more complicated than just, "we're all looking for our one twue wove." Even beyond literary misinterpretation, there's...18 frames in that comic where half-circle looks incredibly stressed about not being able to find its other half. Half-circle isn't even really interested in interacting with any of the other shaped people beyond yelling at them and checking out what shapes they've got on their chest and whether they're compatible (oh, the depths we could go into on this one...I'll spare you the gender studies dissertation).
In real life this translates to what gets termed as serial dating. Monogamous people go on date after date after date, and somehow keep an intensely complicated list of qualifications for "the one" inside their head. For me, it seems a lot easier to just meet different people and let things evolve the way they will, without trying to cram a square peg into a round hole, as it were.
Reason #2: Women are from Mars
This one is tricky, because there's a part of me that agrees with it. Women and men are fundamentally different in some ways, this is true. Does that mean that coupled life should mirror a hilarious set of sitcom set-ups where the guy is constantly putting too much dish soap in the dishwasher and the lady is always drinking wine with her BFF and complaining about how her significant other doesn't "get" her?
This mindset exists sometimes in poly relationships as well, but I see it a lot more among my monogamous friends. Maybe it has something to do with actually having to talk out your limits and your expectations and your rules - it sort of evens the playing field from "man" and "woman" to focus more on the relationship as a whole and keeping whoever's in the relationship fulfilled. It just seems tedious and torturous to have to waste so much mental energy internally deprecating the person you're with all the time.
Ah, tell it like it is, Ewan.
Reason #3: Cheater cheater, pumpkin eater
This is another energy waste activity - constantly worrying where your partner is when they're not with you, and being constantly afraid that they're going to run off with the next person they so much as give a second look to.
In addition to that, I don't know how anybody keeps their stories straight if they are trying to cheat on their partner. Beyond the whole, "don't you feel guilty for lying to somebody who cares about you?" thing, I don't know how you remember if Tuesday was the night you were out for poker, or if Saturday was when you were supposedly visiting your grandmother. I give a lot of credit to monogamous people who can keep track of all the different places they've actually been, let alone all the places they're pretending to have been to cover up relationship indiscretions.