[p_l]

Because screen tearing is something that honestly showed up pretty late, around the time of i3/i5/i7 with IGP, as before that tearing was mitigated at least a little with VSync. (There's the issue that X11 didn't expose good sync primitives, but that's workable with extension, not total replacement)

[dontlaugh]

Even with vsync it happened on X11 on dedicated GPUs. Plus a number of other buffer management artefacts.

It didn’t help that the drivers for everything were closed course, but even with perfect drivers X11’s design can’t deliver perfect frames.

[p_l]

The design can deliver perfect frames - it's extensible so pretty much you need to provide a way to handle double buffering right - and guess what, extensions for that were proposed (Present extension by Keith Packard, building on top of X Synchronization extension)