Look at it this way:
Video hosting is expensive, especially high quality hosting. xHamster and other free tube sites are... free?! Where do you think they'll get the money from?
Producers of Malware actually pay such video sites a small amount per infected machine, because this allows them to advertise all kinds of crap, or even randsom their machine to get rid of it. There's ofcourse a very small chance that people actually pay for such nonsense, but if you have a million views, that very small percentage actually gets a large number.
This is the business plan of most of those sites, as easy as that, because regular advertising in the adult industry hardly makes a penny, and certainly won't cover for the hosting expenses.
If you take paysites for example, they don't need to deal with this at all, they get paid per download, and hosting fees are already calculated in the price. There's absolutely no reason for them to include malware. In fact, that would hurt their name.
So, if you want quality: go for paid sites.
Don't expect free sites to just give away free videos for nothing, that's a very bad business model and they'll be bankrupt within a day.