Running systems is a different set of skills than building them. You can mitigate this to an extent with a PaaS but eventually you'll run up against operational issues that require expertise to remedy properly. You can try to use generalists to handle both produce development and operational roles but you'll start accumulating inefficiencies rather quickly (i.e your entire dev team needs to understand the operational tooling so they can be on-call, you're going to be spending more for resources you aren't using optimally, and you'll likely start to accumulate toil, downtime, performance, or security problems). So specialists tend to pay for themselves pretty quickly.

Even just having separate reporting chains is helpful to achieve a balance between work driving operational metrics (stability, performance, security), and product development work driving product metrics (sign ups, sales, acquisition costs, etc)