Cost, experience and for the paranoid (right or not) control.
The biggest downside is initial cost in time, effort and cash compared to typing in a credit card.
Ok other downsides include lack of power redundancy and decent networking which are more common in data-centers.
Other side of this is, why buy 8xa100 for that project to stick them on eBay to recoup cost when you can rent them?
Agreed - Effort/Cost/Time is what always nips my self-host projects out of the gate. I start working down the recursive thought experiment of everything I "need" to get an email server working (for example) and bail when I see the list.
Convincing the family to buy in is hard too because (as you put) I can't promise the same level of redundancy/service guarantees.