Orange man got it.
Work benches don't gain levels on their own. They upgrade by building the proper accessories next to them. The work bench for example has 4 unique accessories, you need 1 of each to reach the max level bench:
https://valheim.fandom.com/wiki/Workbench

Work stations in general have strict gated progression and you will hit a wall until the next tier of tech and materials are unlocked.