By the time they got rid of the AT form factor found around the pentium and early socket 7 era, motherboard sizes and screw hole placement started following the ATX specifications which meant standardization. Manufactures still sometimes did really dumb shit with case designs but they still do that today. For example I once saw this shitty compaq with the psu right over the cpu so you can’t fit a serious cooler. And those iconic Windows XP Dell Dimensions everyone had were only big enough to fit micro atx motherboards even though the case was basically mid tower sized. I can’t even remember how they made such inefficient use of the space but it involved lots of stupid brackets and screws in idiotic places.
A lot of prebuilts from that era made up their own case dimensions.
Add non standard power supply sizes to the list.
By the time they got rid of the AT form factor found around the pentium and early socket 7 era, motherboard sizes and screw hole placement started following the ATX specifications which meant standardization. Manufactures still sometimes did really dumb shit with case designs but they still do that today. For example I once saw this shitty compaq with the psu right over the cpu so you can’t fit a serious cooler. And those iconic Windows XP Dell Dimensions everyone had were only big enough to fit micro atx motherboards even though the case was basically mid tower sized. I can’t even remember how they made such inefficient use of the space but it involved lots of stupid brackets and screws in idiotic places.