EmmZ, there is no "need" to evolve anything. Either it happens or it does not. So the real question is not why it happens, but rather how.
The answer to the latter also helps to explain why ancestors to extant species (unless under very exotic circumstances) do not exist anymore.
Just to clarify the definitions. If we talk about speciation, we mean that two gene pools do not mix anymore.
Now let us envision a speciation event. We have first the ancestor species, let's call it A. Then due to some events (migration, flodding, whatever) population A splits into to two sub-populations: A1 and A2. Both diversify but do not exchange genetic material anymore (due to the separation). After sufficient generations bots subpopulation may have developed in such a way that they are unable to interbreed anymore: speciation has happened. The subpopulation A1 and A2 would now be classfied as two new speices: B and C, for example.
As you can see from this example, there is no separate ancestor population per se, as both split populations are subject to evolution themselves. However, it is quite possible that one of the two subpopulation (A1 and A2) stay genetically similar to their ancestors (the "whole" population A), nonetheless, logically they cannot be their own ancestors from a phylogenetic point of view.