Is preschool necessary?

I'm Canadian and growing up I don't remember this being a thing but when I moved to Texas (my husband  and children are Texan) I discovered it was huge here! It's like every kid goes to school sometimes as young as a year old!? 
Is it really that necessary? My son is 3.5 and knows his alphabet upper and lower case, can count and identify his numbers up to 20, knows your basic shapes and colors, and I just started teaching him the various states. I'm a stay at home mom so it feels silly to pay someone to watch my son when I'm home anyways. Am I missing something? Is kindergarten more rigorous these days?