I've noticed too many reviews of gear using slap only or the majority of the time, when the majority of the time it's not used, thus not getting a good representation of use in the real world.
Notwithstanding this is a perennial complaint here, perhaps you should change the thread title so that it expresses your real point.
Based on the body of your post, your question is not actually 1. Why do people slap when demonstrating bass gear? Rather it is 2. Why do people use slap technique principally or exclusively when demonstrating bass gear?
The answer to 1 is obvious: Because slap technique is widely used by bassists at large, so its inclusion in gear demonstrations is representative.
The answer to 2 is likely: Because meme culture and wanking.
