If you're interested in this question, you're interested in the field of Consumer Behavior. A area that is studied tremendously by companies, psychologists, and academics at business schools.
When you say you've never purchased something because of celebrity spokesperson or celebrity enforcement, you're in huge company. Indeed when consumers are survived about this, "celebrity endorsement" ranks very low on impacting consumers decisions. The highest factors are things like reviews by experts (eg Consumer Reports) and recommendations from a friend (and, quite amazingly, consumers typically believe friends about the same as they believe experts -- which is totally irrational).
But, the truth is, celebrities enforcements DO WORK -- and actually they work pretty well. Celebrities are good for a few things, but two most importantly. First, they help elevate products into consumers consideration set. Humans can typically only remember 5 +/- 2 of any product in a product category, unless they're an expert in that product. Meanwhile, there are often tens, if not hundreds, of products that the consumer COULD choose from in that product category. Celebrity endorsements help elevate products into that 5 +/- 2 consideration set, which is a a huge advantage over the 90 other alternatives that didn't make be set.
The second thing celebrities are really good at is linking a product to the associations consumers have with that celebrity. For instance, there's nothing inherently "extreme" about Monster Energy Drinks -- I mean, what makes a beverage "extreme". But, its links to extreme athletes creates that association, which is important for the products positioning and the consumer it targets. The most important association with celebrities is LIKING them. Yes, believe it or not, in the not-immediately-conscious workings of our brain, we will take the products linked to celebrities we like, and actually associate with liking the products. One of the more curious illustrations of this is voiceover. Ever wonder why the commercials using voice over don't often say who the celebrity is? That's because it's actually most effective when you DON'T know the celebrity, but you recognize the voice. If you hear the voice, recognize the voice, know you like the things associated with the voice, but don't know who owns the voice, you mentally tag the product with an "I LIKE" association, and don't dilute that association with anything else about the celebrity. It's really quite fascinating.
TL;DR: Most people agree that they don't take celebrities enforcements seriously, but celebrity endorsements actually an effective marketing tool. You're more likely to remember and consider a product of it has a celebrity endorser, and you'll "subconsciously" tag your associations with that celebrity to the product.