Why? I've made all of the patch cables in my house with solid core and never had a problem.
Well they need to be crimped with solid core RJ45 connectors, as the "standard" connector is designed to punch into the middle of stranded cable with a point. Using these points on solid core can lead to poor or finicky connections. So that's the first hiccup people run into.
The solid core isn't made to bend regularly and won't hold up to repeated unplugging, storing, replugging. They'll get brittle and weak in the middle of the cable where it was bent. So your cables degrade rapidly with little indication.
Cat6 specs are so tight, it's extremely difficult to crimp patch cables to code. And they're a flat out pain to crimp to begin with. When you factor the cost of the cable, the crimper, the connectors, wastage, it's almost the same cost to just buy the machine made and tested cable. That's not even taking your time into consideration.
Everything I've read says if you really need a custom length or emergency patch cable, that you should terminate it with female jacks and use premade patch cables to reduce network issues. And then you should focus on replacing that cable with a manufactured one asap. Without a proper cable analyzer/validation, bad cables can cause you all kind of intermittent networking issues that are near impossible to pin down.