Exactly; I guess a lot of people just work on projects/jobs and then move on. Once you need to go back to systems you forgot about (things you wrote 5-10-15-20+ years ago), Ruby (in your example and indeed my experience) is a nightmare on speed. The (strange to me) idea that people have that their code won't be around that long, hits me in the face every time a client asks me to 'connect to something made by someone some time ago'; I go check it and it's almost always something php/RoR/(and lately node) built by someone that left years ago and no-one touched it since because it works. When you actually do touch it, ofcourse, it is completely out of date, nothing current works and because of the flexibility of these languages and systems things change way too fast (really unneeded API breaks in packages is something crazy to me; coming from Java, I expect people to keep things backward compatible, but nope; just toss it out!) (especially node npm and least of all PHP which is actually a joy to work with in this kind of spelonking as it is remarkably stable including it's (older) libraries) (and no, not everyone uses a framework with php 'nowadays'; I run into many PHP projects a few years old that are just plain php, more so than frameworks used and that actually does make it easier to jump as plain php still works from a decade ago after update). And then when you have it running on your laptop, it usually is full of these 'corner cases' (lazy programmer input/output handling I would call it) which don't age well (and often hides bugs).

On the other hand, running into Java or C# projects is hardly ever an issue; it's a lot of boiler plate (many (request/response/dto etc) models/entities and layers), but it's readable immediately and if it still runs, it means that all data is validated and I can trust it.