Because you don’t have to. The few hours you used on infrastructure can be better used fixing a bug, polishing/adding a feature or even giving yourself a break

Because you don’t have to. The few hours you used on infrastructure can be better used fixing a bug, polishing/adding a feature or even giving yourself a break so you can be better focused the next day. A Heroku-like platform will literally do the scaling for you. The non-financial cost is that you need to develop your application in-line with their framework/platform. If you make this decision at the start, this cost in practically nil.