That very well may be, but I think we're at a point where the majority of developers started after git came out (since the industry experienced explosive growth just in the last few years). They kind of forget (or didn't know) that it's not that long ago we did monorepos because we HAD to. The tooling to do a system in multi-repos was just not there. It wasn't practical.
Companies like Microsoft, Google and Facebook predates the days where building your company on top of 3000 repos was practical, and they certainly were not going to convert everything if they could help it. Thus, they built an enormous amount of tooling to make it work. It certainly has benefits (and tradeoffs). With similarly advanced tooling to support you, multiple repos also has a lot of very nice properties and scale quite nicely.
To each their own.