And if you fill the engine bay on an ICE car with battery packs during the conversion, the weight distribution will be extremely uneven and cause trouble with the suspension and related components, poor handling, etc.
That "etc." hides "moving the heaviest, most flammable and non-extinguishable part of the drivetrain into a primary crumple zone in front of you" pretty neatly. :D
Plus, as I noted in the weight part, an engine in a compartment is designed to detach and slide down to protect the cell. Can every retrofitter guarantee the same thing for their battery packs?
That's something spewed by people who don't know enough about cars to know they're chasing the wrong criteria. Battery placement is like a 2nd/3rd/4th order problem. You could fit a very respectably battery in the space where the fuel tank and exhaust go and if not there then the floor might just have to get a couple inches taller in the rear row. Not a big deal. Making battery cases to fit those locations is hard, but also not crazy. Just scan it like Weathertech and Uhaul do for mats and hitches.
The first order thing that's keeping all this from happening is that there's no money in it after all the expensive re-engineering and low volume manufacturing you'd need to do to integrate it into the vehicles you want to support.
This is why the industry is kind of stalled at the "supporting DIYers" level. It just don't work without free labor doing the vehicle specific bits.
You can fit some battery there, but liquid fuels have a much higher energy density and so I wouldn't call it respectable. I have know people who converted a car to electric, and finding places to stuff batteries was the major challenge, they did the fuel tank of course, but then went looking for any other unused empty space. In the 1980s old trucks were favored because under the bed there was a lot of empty space to work with (even then those old lead-acid batteries didn't give much range)