Combat is generally an attack roll and a critical roll.
Spells are usually either a casting roll and a RR or a casting attack and a critical roll. (A few spells like "Vacuum" are Spell Casting roll, RR then a critical Roll)
Picking a lock isn't actually the best comparrison, better would be two characters on either side of a door, one pushing it open, while the other is pushing it closed. . .opposed activity.
Likely the main reason most games include a RR type mechanic is that players are less surly when they fail an RR and go down, than they are when the NPC casts successfully and as a result they are dead.
I suppose, if you wanted to consolidate, you could transpose the RR table to a bonus value for level difference, then apply the RR bonuses of the target as penalties to the base casting roll