For me, because once you get beyond the $5 nano/free tier, things get expensive really quick.

For example, I run about 10 different sites off one Linode, but only one of them gets any substantial traffic. Still, in order to run that site, which works just fine on a $20/mo 2GB/2core unit on Linode, I'll push out about 150GB in outbound bandwidth a month, and require around 15-20GB in storage. Pricing that out on AWS, ignoring the free tier, I'm looking at at least $40/mo, so double the price.

These are hobby sites or side projects, so they are not necessarily mission critical, but it would be sad if they went down.

There's a serious benefit to getting a VPS when you're on the low end like that, where power and bandwidth per dollar are greatly maximized.