1. Because they pay us.
2. Yes.
3. Yes, for the most part.
4. Mostly because the keys being sold (on legitimate sites, anyways) are usually volume license keys, which are purchased at a huge discount vs a single standard Windows 10 license by OEMs mostly, who occasionally sell off any extra keys they have to make back some money if they're not going to use any of the extra licenses. While technically this is against Microsoft's Terms for volume licenses, and could have a chance of being revoked, it's very rare that it happens with proper volume license keys so it's about as good as any other method.
There are some downsides to these kinds of cheap keys, the major one being that as they are OEM keys, once they're activated they're locked to the general hardware that you activated it to (usually it uses your motherboard, so you can still upgrade GPU/RAM/CPU and it'll be fine, but if you replace your motherboard with something different it may break activation). So if you bought or built a new PC, and wanted to transfer you OS over, there's a very good chance the key would no longer work and you'll lose activation. There are some cases when it may transfer just fine, though it seems more hit and miss than anything.
The only time I've seen cheap keys get revoked is when they were bought from more shady sources, like eBay, Etsy, etc etc which ended up being keys bought from stolen credit cards, so when the charges for those keys are reversed by the credit card company MS will then deactivate the key bought with it. However, I've never personally seen a volume license OEM key get revoked, even though it's against MS's terms to resell them.
If you're that wary, you can just spend the $200 MS charges you for a key.
Or you could find an old Windows 7, 8, or 8.1 key, like ones from old laptops or desktops you own but never upgraded to Windows 10 with, and use those to activate your Windows 10 install for basically free. This works because Microsoft still offers free Windows 10 upgrades, even though their promotion for that technically already ended and they don't advertise it as free anymore. No idea why it still works, I suspect it's mostly because Microsoft just wants everyone to upgrade to Windows 10 so they just never bothered to update their activation servers to stop accepting old keys, but it's also a valid way of activating Windows.
