I typically write about 20 letters of recommendations a year. Writing all of those isn't easy, in part because most are due at the same time of year. To make matters worse, every student wants a strong recommendation, but few students deserve one. Sometimes I get a request for a strong recommendation and have to say no.
It seems mean on my part. My letter can get a student into their preferred school, putting them on a good path that will affect them for the rest of their life. And it doesn't cost me anything to write a strong recommendation: I just have to add the phrase "I strongly recommend…" to the letter.
Of course, it's not so simple. If I gave more strong recommendations than I have strong students, my recommendations, strong or not, would mean nothing. Even if it doesn't physically cost me anything, I'm reputationally bound by the constraints of reality.