This was discussed not that long ago -
https://forums.macrumors.com/threads/do-you-want-a-quad-core-13-rmbp.1962544/
It was interesting, I learned from it but the topic goes on ad nauseam, in my opinion.
While difficult, putting a 45W quad-core Intel processor into the 13" laptop form is possible. Sony puts the same quad-core Haswell processor used in the low-end 15" MBP in the Vaio Canvas Z 12" tablet. However, in the thread linked above, nobody could come up with a 13" quad-core laptop currently being sold by the major manufacturers. There are 14" quad-core laptops available that are gaming-oriented. Intel has a i7-6822EQ quad-core processor which takes 25 watts. The problem is that it's 2.0Ghz and has a lower-end iGPU.
So it's technically feasible, but none of the major manufacturers, Apple included, perceives a demand that they believe is worth their effort to produce.