You are playing with words.
Common amongst 24x36 zooms are f/2.8. MZ zooms are typically f/4-f/5.6.
Common amongst 24x36 primes are f/1.4, exceptionally f/1.2. MF primes are typically f/2.8-f/4, exceptionally around f/2.
Sure, you will find some very slow 24x36 zooms and some faster MF primes. But in practice, low light photography is easier with 24x36 cameras. When I want to take pictures of, say, a Jazz concert in a not so well lit Jazz club, I know why I do not take my MF camera.
1.4 vs 1.8 for a 'standard' focal length isn't that much of a difference - and I bet you'll find more zooms in the range of 3.5-5.6 for small format than 2.8.
And there are lots of primes in the 2.0-2.8 range. Look at Pentax 67 lenses, they have 2.8 up to 165mm and 4.0 up to 800mm - even the Canon, Nikon and Sigma 800mm are 'only' 5.6 lenses!
And you can find 300mm 2.8's from a lot of makers.
However I think the whole 'argument' is flawed, the DoF is very small even at 4.0 and a lot of MF lenses are quite sharp at 4.0 - if you had a 2.8 lens with the same coverage it would be too big and heavy to be usable, especially handheld.
Imagine a 2.8 zoom that has to cover 645 or even 67 - that would be huge and uncomfortable to use, DoF would be razor thin - even with a good AF system you'd only get a fraction of your shots in focus.
MF lenses don't need to be superfast. They can be but they don't have to be.