I can’t tell you why they are named that way but you are not the only person who gets them confused for sure. It’s just one of those things you either have to memorize or be prepared to google a lot
There similarly named with similar actions, but if you look up the meaning of the words they have different meanings. slice basically means to chop something up and remove portions, and splice is adding two or more things together. What helps me is thinking of slice as slicing something off, nothing will be added back or combined together from those sliced off sections. And splice involves slicing something up, but ultimately those sliced pieces are going to be joined back together in some fashion.
I still usually forget that and look em up anyway just to be sure, but maybe it will help you.