It's a bandwith vs latency issue.

Imagine you make 5 commits in 4 hours. You now have 5 builds queued up and 20 hours of latency. So one would probably prefer to only build the last commit, and maybe queue the older commits up for later. But if you make another change then you'd need to pause the build of the older commit, or you'd be at up to 8 hours of latency instead of 4..

If a build takes 4 hours it argues that a build on that system is a pretty expensive operation, unless changes are very infrequent, which in most systems is not the case, a build run on every change would seem to be highly problematic.

That would be ideal, but many organizations don't yet have that capacity for large builds/deploys (yet).