I think your premise is somewhat flawed, analog SLR started to be designed this way with Canon’s T90, about 35 years ago, and there are enough mirrorless cameras (i.e. Panasonic’s G6), and the reason was and is Ergonomics.
To some degree, some mirrorless cameras reintroduced the less ergonomic “retro” design, which was good enough when they were mostly using smaller and lighter lenses.
With lenses for mirrorless also having bigger apertures and therefor being bigger and heavier, their designers are back to realising that having bigger grips is actually helpful
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