Of the 3 motorcycle forums I regularly participate on... 100% of them are.
Some folks like vB.. and if 3.x series is working for them they don't want to have to upgrade, especially if they are severely customized with bespoke add-ons.
You want to pay for all those folks to upgrade?
Some of them don't have the money to but want to continue running their forum so they stick with what they have that works.
You are talking now about developers.. who are NOT the hosting providers. The hosting providers still have to provide to the LCD to maximize their profits.
The developers can do what they want and then have the purchaser go to their own hosting provider. The point is that as long as there are people running vB 3.x series, you are going to have the OLD versions of PHP out there.
That's why I'm on my own hardware - I can control it. BUT not everybody is capable of that and not everybody is going to go pay the price for a "performance" host provider. I can't tell you the number of times I've seen in this very support site folks buying XenForo and then asking about the cheapest hosting they can get to run it on.