Not sure if the Mac Pros are actually built any better, but despite their weight and appearance the PowerMac G5 cases are... not particularly robust.
I was lucky enough to have a free G5 thrown in with a somewhat rough 20" Aluminum Cinema Display I picked up locally a few years back (a Late 2005 2.3, no less)! Unfortunately, the previous owners had moved houses without putting it back in the original packaging which had heavily damaged the lower case, since there's not much attaching the bottom feet to the side of the case under the access panel (some small rivets and some sort of heavy glue IIRC). It had come fully detached and was sagging at an angle under its own weight.
Thankfully, my dad's a retired mechanical engineer, so I brought it down when I went to visit and we managed to bend and re-attach the case to be in passable shape with some pop-rivets and JB Weld:
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Despite this, we weren't able to un-distort the metal to be completely level again so it's *slightly* lopsided, which I've addressed with some dollar-store furniture pads that make it look pretty normal at a distance:
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But yeah, these sadly weren't built to withstand much physical abuse (at least, not to the bottom case). Luckily for mine, I have no intention of physically abusing it further.