I don't think that it's necessarily an outright bias towards making the wizard stronger, but rather that the wizard is an extremely wide concept, coupled with the historical design of the wizard, having access to all of the spells under that wide umbrella.
Imagine if you will, if the 1e wizard has been much more specialized. There would have been a much more limited pool of general spells shared by all wizards, and beyond that wizards would get spells from their specialized schools (necromancers would primarily cast necromancy spells, illusionists illusion spells). There's no "generalist" wizard in this paradigm.
I think it's likely that today we'd have wizards that were evolved along those lines, and therefore far more specialized than the wizards we have. And it would have been the same designers in both cases (albeit from alternate timelines).
Unfortunately, rather than using the 1e Illusionist class as their baseline, the 2e designers chose to use the Wizard, which is why I think we are where we are. If it had been the other way around, I think people would largely accept that as the norm (and probably argue that a "generalist" wizard would be completely overpowered).