because it cannot be done.
currently, engineers at weapons works based on the squad.
each squads has a parameter with numbers of enginner unlocks:

same as radio operators
so in a sense, hard coded, and spaghetti coded.
which, currently, the reason why you see people with pak 40s, it’s because, normandy and berlin engineers had pak 40s. and those were the campaigns that a good 80% grinded.
new players should theoretically be using the pak 38 due to having basic italian engineer squad as default squads, and pak 40s from the other engineer squad.
a very tiny portion played moscow. which results in pak 36s being rare.
which, right now, if you take out an engineer of the moscow campaign and use them on berlin with t5 weapons, you’ll be forced to use the 37mm.
and it’s less than ideal.
giving the br III appearing in T1 matches, it’s best if it remains a pak 40/38.
but then, if you take the moscow anti tank squad ( which it’s also a tt squad ), you’ll get access to the pak 40/38 ( not sure ).
and if you would switch all enginners to pak 36s, they wouldn’t be used very much outside low br by seal clubbers.
from what i could gather, a bird told me that, the reason the merge happened, was to cut loose most of the older squads ( which it’s why we haven’t got medics either to this day to any of the pre merge squads outside pacific and stalingrad ) outside the very few moscow squads that became TT after merge.
the idea was to reduce the number of squads to work on.
and accomodate as most as possible of the newer stuff and rework customization on those.
EDIT.
i made an error in judgement and meant the 53k. which does look like a pak 36 but it’s being used by the soviets.