Only anecdata, but if the team (company) is small and you're chronically understaffed, there may not be enough time for "training" - maybe this is a bit skewed

Only anecdata, but if the team (company) is small and you're chronically understaffed, there may not be enough time for "training" - maybe this is a bit skewed towards "more generalists than specialists" - but I've had this experience. Let me try to give a only somewhat made-up example. If you have 3 people doing a big web application in one stack, I see no problem having 1 or 2 of those 3 be relatively junior. If you work with 5 different tech stacks that keep different parts of the company running, it might be a bit hard to get someone to learn 2 programming languages in addition to needing a little advice in their main tech stack. Not saying it's a valid reason, but if a lot of your hard decisions are actually important architectural ones and not small "how will this small piece of software be developed", senior people are actually a lot more important than usual, than say, in a team doing feature development. Also about your definition of Junior.. depending on where you work, 4 years often is not really junior anymore - elsewhere you'll not progress to non-junior until a few years more...