The different "mentality" is whether sequential execution or lockless parallelism prevails. SIMD instructions in a CPU are very small islands of parallelism that do a strictly limited number of elementary small jobs simultaneously within a single opcode, with trivial synchronization (the next opcode is executed next); SIMT jobs in a GPU are an arbitrary number and arbitrarily long and complex, you don't know whether they are waiting or running or done without explicit synchronization primitives, and in-order execution doesn't extends beyond the boundary of a shader.