[Falkon1313]

Partially because once people realize that they're in dependency hell, and no one can guess at how many thousands of places across 100 sites might be affected by a minor change in one spot, they don't even try. That minor change has become impossible (until the next full rewrite), or at least not feasible within a reasonable budget.

So instead changes must be made outside the generic shared code and you end up with 100 slightly different sites that can't be upgraded.