The architect creating the blueprints of a building usually doesn't do the designing, either. Nor does he build the building.
You have the artists sketching the rough outlines that their customer agrees with. Meet the UX-designer.
Then you have the architects creating the blueprints, the actual real-life product... on paper. That's the UI-designer, or the front-end developer.
And you move on to the builders. The people mixing the concrete, climbing the scaffolding, pouring concrete, laying bricks. Manual labor, specialized labor. Those could be your back-end developers.
Then comes the time consuming part of it all. Making everything look good. Installing decorations: glass, decorative wooden panels, installing the kitchens and making them work, installing the bathrooms, the floors, doors, windows. And hell, even placing pots with plants in them here and there to make it look good. They consider the flow of the people going through the building: it needs to perform, it needs to look good, it needs to work for people of different sizes and speed. That's the front-end developer.
Lastly, you have the building inspectors. They see if things are according to specs all along the building process. They check for safety guidelines, they reassure everyone involved that it's safe. Those are our testers.
Sure, you can do UX and UI and the back-end and front-end development. You can even do all the testing.
I can do all of it. I truly can, I'm a good designer. I could learn UX in a few days. Back-end? Done that, I know a few databases. I know how to setup a server. I can measure server performance and pin-point bottlenecks. Buuuut I'm not a specialized professional at any of those points.
I specialize in front-end development. I know HTML and CSS, and I know client-side javascript. I know of performance, I know about browsers, I know about composite, paint, and layout. I know about the developer tools available. I know in-depth things about front-end frameworks.
My back-end devs don't.
My designers sure as hell don't.
They specialize.
I specialize.
You should specialize.