From the old days where people produced everything by hand and were closely specializing in doing the whole process by themselves, we got up to the point of massive production and people getting singular repetitive jobs which lowered defects and improved quality. There still is a reason for this as doing everything by yourself is very inefficient and by doing this your business is bound to failure.
I remember an anecdote where a boss got a phone call from one of the employees wanting to move up to a manager position. He asked the employee what he was good at and the employee said: “Everything”. The boss laughed and jokingly said: “Well, this company only need one person who knows everything, and that spot is already filled, so I guess we don’t have a spot for you.”.
This funny anecdote represents the problem quite accurately. True efficiency and quality is gotten from people doing their jobs and knowing their jobs, which leaves room for constant improvement. That is why it is very good and is my personal advice you specialize in any part of the industry, whether that be UX/UI for mobile devices, web sites, typography or whatever you find best and you believe you can utilize your skillset best.