This just seems bizarre to me. A hag is a monster that needs to be a lot more than a combat encounter. Why buy a book if important rules are just going to be left to GM fiat?
Mostly, lack of space. If you have Volo's, you see that hags got an entire chapter to themselves. Back in in 2e, there was an entire Ravenloft supplement dedicated to hags. If you wanted to give each monster the full amount of space it deserves, you'd need a
much larger book. Or you'd need a bunch of MMs, each with only a handful of monsters in it. That would actually work well for a different system, but D&D has always had a metric ton of monsters and nobody would be pleased if they had to buy five books just to get the standard critters.
And also, it's because a lot of people aren't going to run monsters as more than combat encounters. Sometimes really cleverly-done combat encounters, true, but not much more than that. A typical, if simplistic, adventure might be: There's Bad Stuff happening in a village. The PCs discover that the Bad Stuff is coming from the Deep Dark Forest. The PCs investigate the Deep Dark Forest and discover signs of a hag. They defeat the hag's minions, then eventually defeat the hag herself. The End. They may also have the potential to ally themselves with a Good Fey or other forest-dweller who by itself is helpless against the hag but provides valuable assistance to the PCs.
Which means there's two ways to figure out the Bad Stuff: by (a) combing the books to give the hag all the spells and magic items they need to accomplish what the DM wants her to do, and then rewriting the statblock so she can cast those spells; or by (b) a judicial application of handwavium. You want the hag to be able to blight the crops? Does it
really matter how she did it, in terms of level and school of spell or whether she was attuned to a magic item that did it? Or is it more important that she blighted the crops, and that the PCs can possibly reverse it in some way?