Personal training can get pretty personal.
When you’re one-on-one with someone, you have more time than you know what to do with. You can test-retest, make up stuff on the fly, and think up new cues that you can use with later clients.
When you have 30 people in a big group, you aren’t doing any of that. sure, you can make up some new cues or whatever, but can’t really see it through like you can when your one-on-one. Do you know if it worked? Too late. It’s time to coach the next person.
Large group training has its difficulties, but it’s also a really efficient way to train certain types of people. If you have people who are out of shape and not in pain, get them in shape in a group. You build camaraderie, forming team through shared suffering. and even if they’re already fit, a group workout is a great way to keep pushing everyone together. Community is one of the best ways to build fitness inertia.
But, if you’ve coached for any length of time, you know that not everyone fits into these simple buckets. I can count on one hand the number of clients I have who don’t have some sort of chronic pain issue that they’re dealing with. I don’t usually recommend that they join these groups, even though and they’re free to take over here at Google and they have some very competent coaches.