Your children do learn so many valuable life skills at pre school!
Me and my siblings all went to pre-school - We learnt a range of things, but not too much academically (no reading, writing etc) but the option was there. It was play focused, but if we wanted to try and write our name/read a book, we could and the teachers would help us. We learned academically at school (reception onwards) and could all write half of our names by aged 4ish (we all have quite long names), read basic words and write and understand numbers, shapes, colours.
We also all loved school with a great group of friends.
My friends children didn't go to pre-school. She stayed home and taught them to read/write/numbers etc. They knew all of it before reception. She really brags about how intelligent her children are and how they're way above their peers. She said 'My 10 year old is working at A level grade in all subjects. Teachers say if they were to sit a GCSE now, they'd get an A* without a doubt' - That's great and everything, but her children also don't have friends at school. They can not socialise. They can't deal with arguments (he gets his mum to sort it), they couldn't eat without spilling food until they were around 7 years old, couldn't cut paper/food, didn't know how to listen or take instruction from anyone other than mum until year 2. You get the idea.
He's very bored. He finds school boring because his work is too easy and he doesn't have friends. They're lovely children, don't get me wrong, but it's all academia with them. They don't really know how to be children if that makes sense. They had stopped playing with toys by aged 6 etc.
Your child may not be learning academically, but that doesn't mean they're not learning. Pre-school often teaches some of the most important life skills.