Part of that is that we are reaching the point where a plug-in hybrid is actually more expensive to produce than an EV.
Go to Toyota's website today and you'll see that a Rav4 Plug-in Hybrid is more expensive than the bZ which is the equivalent full EV.
Going to an EREV layout (a series plug-in hybrid where the gas engine cannot drive the wheels mechanically) doesn't really save any costs directly, but allows sharing the platform between EV and EREV models.
An EREV is basically the worst of almost all worlds. It's a BEV with a motor tacked on. It ends up with a battery not a lot smaller than a pure BEV, so no real reduction in the cost of an expensive component, plus an engine and generator along with associated maintenance headaches. Not to mention the almost certain loss of cargo capacity.
I can't see myself ever wanting a Lightning EREV, for example, because of this exact issue. If I needed to tow, that would be the sole situation where it might hold enough of an advantage to matter. Aside from that, it will have (most likely) about 75% of the battery the BEV Lightning has, and it will lose the frunk, which is one of my favorite features. Plus, unlike my Lightning with it's 200K mile first required maintenance work, the EREV will probably have to be serviced every 10-20K miles. Ugh.
Yes, towing is a big potential case for EREV. A 600 mile EV truck (300 when towing) would need a 250kWh+ battery pack.
An EREV version could get by with 50-100kWh, which is also a lot less weight to carry. And would give plenty of power for driving EV-only for shorter trips when not towing.
The Chevrolet Volt is a representative EREV. Its battery is 1/3 the size of a Model 3's -- that's a lot smaller than a pure BEV. 50 mile electric range, 45 mpg on gas. These cars are now 10 years old (approaching 15 for the first generation), and out of the 3 known common issues for Gen 2 only one has anything to do with the gasoline engine.