[dagw]

Uhm, why is this bad?

There are two big problems. If someone has lived without X for long enough they've probably found workarounds for living without X so that just adding X might not be that valuable to them or at least it might be hard to convince them how valuable it is to have X. The second is you often greatly underestimate how much work it is to get to the point where you have a good enough clone to even have something to add X to. And while you're busy working on your clone the original company might get around to adding X and thus undermining your whole business.

Haven't we all seen examples of companies that do the same stuff as other companies but better and succeed?

Sure, but they're often better in a whole category of ways (price or speed or ease of use etc.) and not just a clone of an existing product with X added.