As long as you are not your own boss (and even in this case, tbf), you are likely to not do only what you "want", because you are not the one defining what is your job.

Honestly, it's a weird remark. Pretty much all jobs on this planet have aspects you wish you don't have to do. You work for your employer and your employer needs you to train the juniors. If you don't want to do that, change for an employer who only recruit senior engineers, if you can.