It’s not laziness. It’s intelligent product management: Improving the experience for 80% of users by sacrificing the 20%. The integrated chip solution which disables GPU support is what enables significantly better performance for users that don’t need it.
80/20 split is likely an order of magnitude off. This is mainly more about software than hardware. Apple has eliminated dGPUs for all product models from iMac through upper end MBP. By volume shipped, Intel was the dominate mac GPU vendor by far. If you sit in these macrumors forums you'd think there was a vast AMD vs Nvidia war going on. That really wan't the 'world war' at all if take two steps back and look at the whole 'forest' rather than just one 'tree'.
If loop in the iPad (**) with plain M-series now the 'experience' is more like 99.96% vs 0.04% even if Apple had looped only the Mac Pro (plus some odd ball eGPUs that were left) in. If round that to one digit after the decimal point its 100.0% to 0.0%
I think Apple's primary objective was to wipe out dGPUs entirely from the laptop line up. They did a n extremely good job of that. The collateral damage on that though turned out to be wiping out dGPUs on the iMac performance class of Macs also. Three years ago if told folks that Apple was going to match on some benchmarks the 4070 with an iGPU , you would have gotta lots of "what are smoking" looks.
** Point at the iPad saying those a iPadOS apps and mac has macOS apps completely misses the point here. If you drop down to just the graphics programming level of those apps and put aside the relatively narrow UIKit, File storage libraries, and a few other orthogonal library differences of those apps, it is going to be mainly the same optimization changes need to make from the legacy ( 10 years ago) graphics structures where had to copy data to/from the GPU and several other baroque issues that Apple GPUs don't have. That GPU programming subset if what Apple is trying to put "all the wood behind one arrow" . Extremely optimized apps in this specific area should work better all the way up and down that hardware stack. That shared 'experience' is what they are managing to.
It is mainly software ( and the assumptions built in that hinder/help performance ) that is the primary issue.