Or they—like my company—have complex code bases in complex languages (C++) that mean that you're not going to be an effective developer for anywhere from 3 - 6 months after hiring. It has nothing to do with training you for our specific needs, but everything to do with the fact that in a mature codebase there's lots of moving parts that can easily be broken, and there may be older choices that may not make sense because you weren't around when the choice was made.

Candidates with short-term employment backgrounds aren't a problem, per se. We want you to be a long-term employee because you like it at our company and want to contribute to something meaningfully. We don't want to be a short-term paycheque job.