This is because the people on TGP aren't playing actual club gigs like real musicians play. They pretend that's what they are doing, but they are fudging the truth.
"I take my 15 watt combo to the slam poetry/open mic night at the coffee shop" is an entirely different universe than playing real venues and they know it's bad optics, and will reveal they don't know what the fuck they are talking about.
Unfortunately for them, the "grab n go" amp thing is the most reliable tell in the universe that they:
1. "gig" at open mic nights where most of the other "performers" are doing comedy routines and juggling
2. Play at the county fair once a year where the PA is going to be massive/overkill, and not one person there is to see you play
3. Are literally a rockstar and can use whatever amp you want because you have your own sound reinforcement in your tour bus
And simply through occams razor, you know that 99.9999% of those guys are number 1.
Yes, some small clubs will have reliable PAs and monitoring situations, sure, but literally any gigging musician will know that's playing with fire to depend on the venue having it.