Because a motherboard is primarily designed to accommodate a specific CPU footprint only those CPU's that are built using that same "footprint" will for their contact points will physically fit into that CPU socket. A simple example would be a glove with 5 fingers, and for the sake of this example, you have 6 fingers. Only one of your fingers can go in a finger on the glove. The answer is, the glove won't fit you. It's the same with CPU's. If a CPU has fewer or more contact points than the socket it simply won't fit physically or electronically. So and AMD cannot be put into an Intel socket because the footprints are different. However in some cases within the same brand, such as Intel, you "may" be able to change out the i3 with an i5 or an i7. The same holds true for the AMD brand. It's all keyed on the footprint of the LGA (Land Grit Array of the contacts). For a quick example recent generations of Intel CPU's have used LGA 1150 and are currently using LGA 1151 so the footprints for those two generations are different from on another. I'll leave it at that, since making my answer much longer will probably make the mental image cloudy. Google LGA 1150 and LGA 1151 and you should be able to see the difference in the pin/pad locations. Hope this helps.