Why is it when (so-called) famous people are doing?

The whole idea for doing that is for the advertiser to reach out to as many people as is possible, & from them, some percentage of people will 'believe' that the product is really used & favoured by the celebrity endorsing the product. If your next door neighbour were to endorse a product, the attitude of people would be one of "who cares what they use". But if a celebrity endorses the product, people believe it to be something that will work, even make them like that "famous" person! It's the same as using sex or bikini clad girls to sell cars. It's just about what attracts the average consumer to pay attention to the ad & buy the car. Have you ever noticed how the volume increases when an ad comes on. It's to make sure that you do listen to it. The same reasoning is behind those ads which are downright ridiculous. Something so silly will get your attention & the underlying message to buy that product will be listened to by you, even though you think the ad is a silly one. These days, it doesn't matter so much to describe the product as being better than others. The important thing is, to grab & hold your attention to that ad!

Oddly enough, the world doesn't revolve around you. People older than you have more money to spend on products. The advertisers are using people that those potential customers might recognize. For example, if you're trying to sell a cream that fights wrinkles, it would be effective to use Taylor Swift as a spokeperson because she's too young to have wrinkles. But it does work to use Jennifer Anniston or Julia Roberts, who women in their 40s know and want to look like.

Advertisers would not pay famous people to endorse their products if it did not work

Perhaps you are not their target audience - they want successful, educated, articulate, high achievers with plenty of cash to spend who aspire to a lifestyle that such celebrities represent. So, no, not anyone can be in their ads.

Evidently, they are endorsing a product. A company would'nt use celebrities in their ads if they didn't feel they were successful. I don't like them either, but obviously some people do.