the fact that essentially 100% of big tech companies use monorepos seems like evidence that it is at least possible to do it in a scalable way...
Today, not quite. I work for a multi billion dollar tech company and we have several thousand repos (and it's awesome)
Git cannot checkout sub directories and it slows down exponentially with the number of branches. It's the opposite of what is needed to run a mono repo in a large company.
The big companies that predate git and such used monorepos because that was the norm at the time, and it was easier to do with the tools at the time, and as they scaled, they just scaled their process instead of changing everything. But several large tech companies, especially newer ones, do the multi repo approach.