Shit sneaks up on you. By the time you really notice it, a huge amount of loss has taken place. It's an optical illusion because a lot of density is lost, but there's enough standing to hide that fact.
Like cutting a ton of trees from the forest, and leaving just enough to make it look like it's mostly still full.
Thats why 8,000 transplanted hairs can sort of look like they've replaced what was 100,000 before loss, by a top surgeon. But really, don't count on it ever looking nearly as good as what you had. Best to preserve a head of hair that's still worth preserving.
You can also grow back a decent amount with most prevention protocols, if you do it soon enough to rescue the follicles that are in a "coma", but not completely dead yet.
It's really basic.
Dut (or fin) and minox do 95% of the heavy lifting of preservation and recovery.
Everything else can help a little, but not much.