- It's a Gamble
- It's presented as the default.
The first of these points might be the strongest of them all. Older editions had you always roll your hit points which means that people who are used to old editions may feel a certain nostalgia to roll them rather than take a more optimal average. Nothing wrong with this, it just carries more risk than taking the average (and less you should end up with slightly fewer HP, about 10 over the course of 20 levels, though that's probably not enough iterations for it to really show up that starkly, there's a 16% on a d8 --better on higher HD, worse on lower== chance you'll actually roll better over the course of 19 levels).
There are reasons that Las Vegas, Atlantic City and now casinos all over the US stay open, and make money (recent AC closings aside). People like to gamble. They don't care that they lose eventually, they are willing to sacrifice certain long term loss for potential short term gain. Considering that a PC's life is essentially a short term game. There is a possibility (again a bit under 16%), that you'll actually roll better than the high average with a d8 HD. This chance actually gets better with a larger HD (more numbers on the die over the high average), and worse on a lower HD. Here are the numbers for d6-d12 (to get the %, compare the "At Least" number to the high avg so 19*(4,5,6,7)+1):
So those odds are what you are banking on when you roll HP. If you play the lotto, those look like pretty good odds. If you like blackjack, not so much.
The last point is also quite significant. Rolling HP is presented as the default, with using the high average a suggested alternative. When you present something as the default, that's what people are going to use.
Looking at this, there are reasons to roll HP, nostalgia, gambling and text presentation are all strong reasons. If your players want to roll, I would make sure that they understand the risk (show them the table). Also ask them about their reasons. Ultimately though, what I do at my table is ask. Do you want to roll your HP or do you want to take the high average? Some players choose to roll and gamble, others choose to take the average. It's up to them, and it can be the difference of a lot of HP, but on average this is 10 HP over the life of an adventurer, that's not enough to take this mechanic out if it's fun for folks.