see, i'd say you're making assumptions about what form a 'pact' needs take, that there are always concrete terms stipulated and agreed to, the initial pact needn't be an obvious transaction, a warlock-to-be could perform an ritual for power that's light on the specific results, carrying around that old amulet from the attic that's your good luck charm, or a whispered prayer in a moment of desperation silently heard and answered, all without an inkling of there being anyone on the other end or that any of those are serving as tacit agreement to a deal with more strings attached than an industrial loom.
and what does the patron get out of it? why their soul of course, in a measly 60* years or so (geez, these mortal lives are so short, aren't they?), maybe even sooner knowing the dangerous hi-jinks some of these people get up to, so let that mortal run around having their fun, the more powerful they get then all the more valuable their soul will be when they finally breathe their last and it falls into their lap.
*yes i know longer lived species exist