Dumping int can be dangerous given that we might be fighting alot of mindflayers during the campaign. Even the intellect devourer which physically is not very....strong. But its ability? Is terrifying. It can drain someones INT and teleport inside their brains. Instant killing them. Oh and then it handles the body like a flesh puppet. Yikes! Some spells target INT and some skils use it. Seeing through illusions or investigating objects for example use INT as part of the skill. So dumping Int on an entire party can be dangerous or a pain in the backside in certain situations.
Mind you most of the spells that target int are either illusion in nature or spells that give deprimental effects like the intellect devourer I mentioned earlier. Confusion I thinka also targets Int. I prefer dumping CHA rather then int on characters that dont have a CHA focus and wont be used to talk to people.
Anyway as for why INT based spellcasting is so rare. I dont know? I think it has something to do with previous editions. In ADND and up to 3.5 spell casting was INSANELY powerfull and dominated by INT based spell casters. Clerics and druids were obviouslty also a thing but a single mage could wipe a entire adventuring party if they were ill prepared. A cleric or druid could also do this, but not the easy degree that a mage could. I think WOTC were trying to correct this powergap and moved a little to far with the balancing and resulted in generating like half of the spellcasters to be cha based instead.
