The certificate remembers a completed course while the skill asks for continued use.

Paper does not decay and ability does. A certificate records that you could do something on a particular day and holds that claim indefinitely, while the underlying skill starts fading almost immediately unless it is used. The gap between the two widens quietly, and nobody sends a notification about it.

Look at what you are formally qualified for and honestly rate what you could still do tomorrow. Wherever the answer is uncomfortable, arrange one small occasion to use it: a refresher, a practice run, a task you would otherwise pass on. Keeping a skill takes far less than acquiring one did.