shmooopy One ad can bring (if my memory serves me right) about 0.002-0.005$ worth. That means, reading a single, short chapter would require you to watch 6x0.02/0.0035=0.12/0.0035 a bit over 35 on average.
Ads are not as profitable as you think they are. Stuff is not free. People have to work hard to pump those chapters, so its only logical that they need to be paid for their work. If you want a candy and don't have money to buy it, you don't get the candy, yet WN still offers you to read up to 3 PREMIUM chapters for free daily. That means that even if you don't have (or don't want to spend) money, you still get 3 candies a day.
And no, all the stories with locked chapters BELONG to webnovel.