The important distinction is that they make the call and you've both gained visibility into your workload and they own the outcome. If it's a bottleneck that's causing pain they'll quickly become aware of it.

There's a balance -- you can do more with more hands, and they need to be well trained hands. But if they're not your direct reports, ultimately they're not your problem. (This also means that you will need to be responsible for the outcomes of those you mentor, and you'll need to learn how to let slow learners go. Oh noes, you're becoming management! It's almost as if there's a reason that good managers with domain expertise are hard to find.)

I guess it comes down to, are you going to be a "helpful" "mentor" or are you going to accept formal responsibility for the success or failure of mentees (vis-a-vis grants, contracts, successful projects, etc.)? Instead of being a martyr, it seems that explicitly accepting or denying responsibility for people who eat up your time (within reason) is the best long-term strategy. You can still succeed without doing this, and you can still fail while doing this, but the odds are in your favor if you demand accountability. (Also, there's nowhere to hide if you aren't as good as you think you are; I think that's a feature, rather than a bug, but opinions may vary.)

The issue in a corporate setting is you are expected to make up for other people. Same in college.

Sorry but almost all forms of failure have individual liability. Merely knowing this helps a lot of people take their situation seriously and do something about it.

When key information regarding how you're supposed to do this thing or where that thing is located or how you're supposed to test Y, or how should this particular bit that wasn't communicated in the user story but was talked about in a meeting you weren't at be tackled, or whatever is locked inside people's heads you need to consult with them to get that information out of there.

And yes having information locked inside of senior employees is a bad idea for the company overall, but that's been the case to some extent at pretty much every company I've worked for.

Well there's your problem, it's a classic multiple-bosses insanity when anybody can put a new task (hire) on your plate. Furthermore, it may also come as bad news to you if you've already been "promoted" to company trainer, informally.

Talk to your boss about how all of this is getting funneled to you. At the very least you should be keeping them informed as to how your day is spent.

Not to mention, that if it's something they can be trained in once and monkey-repeat ad-infinitum, then there is a near-100% chance that I can automate it.