Sinc
Mr Not That Famous
They say it because it works. Try it out once and you'll get more new subscribers than people who unsub out of annoyance. It's strange what works on YouTube. I had lunch with a corporate channel/network CEO and he told me the most effective way of picking up more subscribers with virtually no extra work is to put a subscribe link in the first line of the description. It doesn't seem necessary since the subscribe button is usually within 2 inches of that link, but I tend to listen to people who get 350 million views per month. Results from our gaming channel indicate he's correct.
I hear you but i find it annoying and in the interest of the 1 other person who may be annoyed by it i wont partake in the practice, i would say "If you want to see when my next video comes out, you may subscribe", if it applies though.