There are MANY MANY differences in RAM. First, you have the speed (DDR4 2133, 2400, 2666, 2800, 3000, 3200, etc.) then you have the timings and CAS Latency (CL14, 15, 16, etc. as well as the other timings eg. 16-18-18-36) and then you have voltage (1.2V, 1.25V, 1.35V, etc.).
On top of all that, memory sellers like Kingston, Corsair, G.Skill, etc. get their memory stock from memory manufacturers like Samsung, Micron, and Hynix. But, the sellers can have different stock coming from two or more different manufacturers. They might not work well together if you had different sticks from different manufacturers. To avoid that you would get a kit, which comes directly off the assembly line together.
*Trying to clarify: You don't buy Micron or Hynix for your DDR4 RAM; you buy Kingston, Corsair, G.Skill, etc. and they are using Micron, Hynix, etc. memory for their memory and you don't really know what you're getting from them unless you do some extensive research. Again, buying a kit form a seller guarantees that the two (or more) sticks are the same.