Because developers have to eat? And my experience with Plex is that they handle all the network negotiation so you can smoothly stream from home anywhere, that’s the core of the service.
Although many projects are offered free of charge, individuals can choose to donate or seek sponsorship to ensure their continued development. Not all freely available software depends solely on financial support, with some projects choosing to develop without any monetary compensation. However, it can be frustrating to feel like the Plex Pass and associated account offerings are being pushed upon you.