Because it’s their job :)
But it’s also their job to protect you if other kids in school think they can pick on you.
Besides, now that you’re all teenagers, you never know when one of their friends might just catch your eye or vice versa. It’s also nice to know that if you end up dating a guy with a scuzzy reputation (of which you are unaware) they’ve got your back and will let him know that he’d better treat you right or he has them to answer to.
An interesting little tidbit here: before Michelle Obama was married, she would bring any guys she was serious about to a family event including her big brother for final evaluation.
Evidently, he could tell a lot about a guy’s character by his style of basketball play (did he cheat or bully? Was he a glory hog or a team player? that kind of stuff) So, Barack passed the test with flying colors and the rest is history :)
I’m sure she endured her fair share of teasing from her older brother when they were kids. That’s just how they are.
But hopefully they’re also an ally and a protection for you in the coming years as well. Big brothers can come in handy in many unexpected ways you will likely discover.