Look at total games played, and record the number. Refresh ten times. I got:
1568586
1548548
1565508
1568586
1548548
1565508
1572059
1560707
1572059
1570094
in that order.

Look at their profiles. They all still play games, and they seem to win most of them.

Even when forcing Firefox to clear cache and refresh (Ctrl + F5), it was the same number every time for me.

There are two answers:

The reason why the same 5 people have been at the top of the ladder, is because they are some of the strongest players in the game. They made it to the top because they're some of the best and they also play a lot (varies based on the ladder).

The reason they dont go above 1850 is because right now there are so few teams/people that are 1800+ that the people who are that high get virtually no points for beating any opponent. For instance I am at 1700 in premade 5s, but even beating a team with all people rated around 1600 only gives around 7-9 points. If I were to lose to that same team, i would probably lose around 20. The people who are rated 1800 would get even less points for winning and lose even more. This makes it basically impossible to move up at all without grinding a ton of games and winning all of them. Even one loss can set you back 5 or more wins.

Even though I'm not at the top, I already earn about 12 elo per win and lose about 20-30 elo per loss. I would imagine if you were top 5, the elo gain would be lower and the loss would be more drastic.

I switched browser to Firefox and tried Ctrl + F5 and it stopped changing. Then I tried refreshing normally and after a few refreshes it changed again. I have no idea what's going on.