I want a lock on my bedroom door

I’m 17 and live with my parents. My bedroom door latch is broken, and I’d like to replace it with a door handle that locks from the inside. My parents won’t let me.

They do knock before coming in, but they usually don’t wait for me to answer, they knock and then open the door almost immediately.

I want more privacy for normal things like getting changed, putting lotion on after a shower, drying my hair, having private phone calls, and just having my own space. My boyfriend also stays over sometimes, so I’d rather nobody accidentally walks in on us.

How can I convince them to reconsider?