Because a joiner makes windows, doors, furniture,stairs etc in a workshop.
A carpenter goes to site to fix and install the various things the joiner has made.
https://en.m.wikipedia.org/wiki/Joiner
A joiner is an artisan who builds things by joining pieces of wood, particularly lighter and more ornamental work than that done by a carpenter, including furniture and the “fittings” of a house, ship, etc.[1] Joiners may work in a workshop, because the formation of various joints is made easier by the use of non-portable, powered machinery, or on job site. A joiner usually produces items such as interior and exterior doors, windows, stairs, tables, bookshelves, cabinets, furniture, etc. In shipbuilding a marine joiner may work with materials other than wood such as linoleum, fiberglass, hardware, and gaskets.[2]