Definitely not 100%. It also has a lot less to do with company size, and more about when the company was created. Before the git and similar tools of the world came to be, managing a single repo was a pain, nevermind hundreds or thousands of them. So (almost) everyone did it the way these big companies did.

Today, not quite. I work for a multi billion dollar tech company and we have several thousand repos (and it's awesome)

Not true. Google, Facebook, Goldman Sachs and JP Morgan, all companies that run mono repos and predate git by very far.

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.

My wording must have been awful...because that's exactly what I was trying to say.

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.

Amazon does not use a monorepo, so you might want to rethink your "statistic".

AMZN doesn’t, unless things have changed drastically in the last 3 years.

yeah by writing custom version control software. Am I going to convince my company to do that (which has like 50k software engineers) probably not.

How many companies have 50k software engineers? Seems like the handful that do, should do whatever works for them. The rest of us can just use a monorepo.