For me, it's not an endurance problem but a budget and level-of-caring one
Budget: Our entry fees are ridiculously high when compared to pro and am-am events. On top of that, I have to pay pro for every dance that I enter (and he won't let me register and not dance them - because it's bad form, apparently). Those single dances cost me.
Level-of-caring: Singles are through the day, usually (often hours before my 5-dance - which at least means my multi is at night, so thanks for that, organizers!). The ballroom is empty. You "win" a participation award, and maybe some coupons/stickers. You're often uncontested.
Seriously, just let me skip them - treat me like the am/am couples who get to just come in and dance their events. I worked really, really hard to get to the open level, and I just want to be treated like the others that did too... and not like a dance-leper, because I happen to be in pro-am.
(I do get that for some people, singles are a big deal and a huge part of their competitive experience. I'm not saying don't offer them, I just wish I wasn't forced to do them.)