It's much easier to get someone to visit a website vs downloading a random binary.
Edit: I don't get why OP was flagged though, it's just a question. Shouldn't offend anyone ...
Fun and profit, the two primary motivators of all seemingly-useless things that exist in abundance.
Ask the Flash game community
Flash games knew their limit and could easily be redistributed. Most online JS games are total hell to save/archive and most are lost to time when their host rips it down.