"I need a website for my bakery". "What's supposed to be on it?" "Our address, opening times, a few pictures". I build them a static website.
"Now I need a contact form". Ok, that doesn't really fit into a static website, but I can hack something together. "Now I need to show inventory, and allow customers to pre-order". A static website won't cut it anymore.
When you develop for clients, especially those that you don't know very well, it's a bad idea to back yourself into a corner that's not very extensible. So from that perspective, I really get why they give plugins such a central spot.