Just get a quad cab truck its the best of both worlds.

And where do you keep the trailer? That’s a lost garage spot and many HOAs don’t allow you to leave them parked wherever outside.

Over here in (Western) Europe most people rent a trailer when they need it, unless you are living more rural. If you need it once in a blue moon then renting isn't really an issue. Many places that sell large things, like IKEA, also rent out large vans basically for free if you purchase from them.

Right, but if you’re willing to rent, might as well just rent a truck on demand. At this point you’ve shifted the goal posts to, “why do people buy things and underutilize them instead of renting on demand”?

Why have a guest room when there are hotels you can send visitors to?

Why have a 4 door car that incurs most of its mileage in a commute with 1 or 2 people?

Well, here at least trailers are rented because you can pick up a trailer at the gas station at zero notice. Renting a truck is a little bit different.

Where is "here?" I've never seen trailers available to rent at gas stations.

Here in Sweden they're available at most bigger gas stations.

That's a sensible way to do things. In the U.S. I haven't seen it; maybe that's partly why pickup trucks are more popular here.

New Zealand

I don't think most people have guest rooms on purpose. More often those are the rooms kids lived in before they left home and then the rooms get repurposed.

Mine is actually on the side of the house, behind the trash cans (also forbidden). The HOA hasn’t messed with me because I let the HOA guy borrow it. Utility trailers are very useful.