Because you will end up investing a lot of time learning skills that don't transfer anywhere else.
Two of the last three companies I've worked at had their own internal version control systems. I'm sick of having to learn how to check code in all over again every time I start a new job.
Yeah. This is one place where startups have a distinct advantage over established firms. Established firms tend to have home-grown build and deployment systems, either because they started writing code before open source versions of those systems became available or because of straight up NIH. Startups, on the other hand, aren't big enough to tolerate that kind of wastefulness. They use as much open-source and standardized software as possible, simply because they don't have the time or manpower to write non-business code.
The last company I worked at had one. How bad of a sign this is depends on when it was developed and why. Theirs was developed in the early days of VCS, when none of the off-the-shelf systems had the feature set that they needed. Probably they kept using it longer than they should have, I suppose out of inertia, but they were in the process of moving away when I left. In that situation, it isn't too bad of a sign.
On the other hand, if you ever find a company that wrote their own VCS 3 years ago that has half the features of Git, then run far and run fast!
Two of the last three companies I've worked at had their own internal version control systems. I'm sick of having to learn how to check code in all over again every time I start a new job.