PCB design of the 80s (Arcade PCBs) - Ground/Power tracks
It was mainly convenience. Having the ICs in a regular grid has lots of advantages, some of which you've mentioned.

With low functional density, any significant application needed a lot of ICs, so the ability to refer to them in documentation by a grid system made for much easier debugging.

You'll notice that ICs of different lengths are still placed on a regular grid, so it's the grid that's important, not the space between them. The improvement in area that you could get by squeezing them together is small, and outweighed by the loss of convenience. It's one less thing to think about when laying them out, the ICs are going to go here, the power grid is going to run in straight lines here and here.

It's not a CAD limitation per se, after all, the CAD copes with local departures from regularity quite happily.

Ye olde 74 series TTL was very power hungry, so boards like this needed (relatively) thick/wide power tracks. LS74 cut that power a lot, and by the time HC cmos came along, processors and logic arrays were starting to reduce the need for boards of dense logic just to get functionality.