Well that's a generalization - there are certainly places where it's not - but it's pretty common at tourist destinations. And there are many "vacation divers" who've never dived any other way!
I was actually quite startled the first time I encountered it, which was in French Polynesia. I was on a small cruise ship that had it's own dive op, and we dove off zodiaks that would take 6 or 8 divers out to the dive site. When they explained that we were all going to stay together and follow the DM, I was like, HUH?
Hey, I'm certified, I've got a buddy, I know how to navigate - why can't I go do what I wanna do? But that's just not how they do things there. I got to know the dive crew pretty well over the course of the cruise, and they explained that many of the divers are vacationers who rarely dive, some not for years. So they just feel safer with everyone staying together.
Our next vacation brought us to Belize, where we were on a larger boat - 12 divers. I thought for sure THERE we'd be able to go do our own thing. But no - they broke us up into two groups, each with our own DM leader. Here they were a little more lax - they let a few of us go either ahead of or behind the group, but we had to stay within view of the DM (with viz of 100+ ft, it wasn't too hard!).
The problem with these DM-led dives is that your bottom time can be limited to the SAC rate of the biggest hoover in the group. This does NOT sit well with me - when I'm paying over $100 for a 2-tank dive, and I end with 1500+ in my tank each time due to gas-suckers, that is just not a good value.
The good news is that there ARE dive ops out there that will allow the hoovers to head back to do their safety stops on their own, while allowing the rest of us to finish our dive. That is really the way it should be handled, IMO. When we were in Costa Rica a couple years ago, we had 8 + the DM. Everyone had a buddy, and when someone hit the turnaround spg they and their buddy were pointed back towards the boat so they could do their safety stop on the line, and the DM continued with us. My husband finished his tank before I did, and was sent off to the line with another buddy team. The DM then continued on alone with me for another 10 minutes - it was great!
I now make a point of emailing with any dive ops that I'm considering using on vacations, to ask what their policy is on that. If they say that the dive ends when the first person in the group breathes out their tank, I won't use them. We've got a trip to another region of Costa Rica coming up in December, and I've chosen Rich Coast Diving. They have assured me that they allow you to use your full tank, and won't make you end the dive if others in the group finish before you. That's a big reason I chose them, because some other dive ops in the area are well-known for ending the dive based on the fastest hoover in the group.
Just a little advice for when you start doing dive travel!