The a12 has about 3-4 times the number of transistors as i7-7700k. I know there are a lot of other on than the SOC core than the cpu for a12, but they still have a pretty large transistor budget. It was made with 7nm transistors vs 14nm that intel was using at the time when 7700k came out. Apple did a good job designing the chips, but TSMC is a huge part of why these chips are so good. For the longest time Intel manufacturing process was way ahead of anyone else which reflected in their performance. If I give you way more transistors I can probably make a better chip.

If I give you more cache, decoders, execution units etc. I can make a faster chip.

Also if you used all cores for sustained amount of time the intel would win out just because it can handle all that heat.