They are proving something for FREE - without ads - you would have to PAY to see the video. Servers, internet connections, and support are not free.
Many are part of public companies - Google owns YouTube and they eventually will need to show some earnings.
Remember the 'free' internet provider NetZero - yea - they had to give that up since there was no viable way to pay for the equipment.
Many websites started as free - providing data or service - but that was done while the 'free' money from VM's or their rich partners ended.
Either pay for the service or get it for free and deal with the ads. There is no such thing as money for nothing - or getting chicks for free. --
"It drops into your stomach like a Abrams's tank.... driven by Rosanne Barr..." A. Bourdain