Because seed oils aren't very good for you. Rapeseed oil used to be used as machine oil until better alternatives were invented. Then it was repurposed, put in our food and we are told it's healthy.
If I have mayo I buy Hunter & Gather olive oil mayo. Six quid a jar but worth it.
Anything is fine for you in moderation. Including moderation. Tallow used to be used as a lubricant and for lighting as well, and yet no one says no to beef dripping roasties or chips...
Much of the "it's good for you" or the "it's not good for you" is marketing guff. The ultra processed food that the rapeseed oil is *IN* is likely way more unhealthy than the oil itself. Look at how butter was vilified about 30-odd years ago, and all of a sudden it's now the bees knees.
Most cooking fats are used here chez Casa Reynard - rape seed oil for sweating off / sauteeing vegetables or for shallow frying things like bhajis and schnitzel, butter for omelettes, steaks and roux-based sauces, lard for general frying, beef dripping for roasties and chips, sesame oil for finishing off asian dishes, and extra virgin olive oil for things like salad dressings, hummus and drizzling.
I don't make mayonnaise as i just don't eat enough of it, ergo I have bought. But when I have made it, rapeseed oil is fine.