when youtube first launched in 2005 there was no ads, so why all the ads now

on youtube, many videos are unwatchable now due to all the stupid ads, it ruins the watching experience...but back in 2005 and the few years following, there was no ads on youtube...so why now?

is it becase big corporations hijacked the site and turned it into a commercial tv station, and took the real purpose out of youtube? youtube was originally meant for ordinary people to upload videos, not celebrities or media companies, they have ruined youtube in my opinion.