Because doing so would require twice the cache and execution bandwidth. It is usually better to take the gamble and fetch just one branch. If you don't want to gamble you could stop the current thread and continue with the execution of another thread (if you have simultaneously multithreading/hyper threading).

In some cases the compiler may decide to evaluate both paths and then "undo" the path that was wrong. This is very common on VLIW architectures like Itanium which have hardware support for this (predicates), but it can also be done with other instruction sets.

That would get exponential real quick. But Intel’s Tremont does have an instruction decoder that’s 6-wide only when decoding a taken branch, since it also starts decoding the branch target in the same cycle.