I can’t speak to CoG’s exact reasoning, but a common marketing reason is to encourage people to upgrade to the version without them. Trust me, I’ve seen my royalty statements here and I’ve developed and published my own games. The amount of money we get for ads is very, VERY small.
As a Commodore 64 and original Nintendo gamer, I understand and sympathize with your perspective, but the landscape has changed. It’s not something that can simply be ignored by devs.
Mobile gamers have been conditioned to think of games as free or low-priced. An analogy that might make sense is Amazon’s shipping prices. Amazon takes a HUGE hit on shipping their items to your doorstep for free. This, in turn, has conditioned the average internet shopper to believe that shipping is either dirt cheap or shouldn’t cost anything. Now people are less likely to shop with competition who can’t afford to take a huge hit on shipping prices. When the biggest and most influential companies set a standard, the public expects that to be THE standard.
When it comes to mobile games, the biggest publishers discovered that the best way to make the most money is to offer the game for free and charge huge amounts of money in optional microtransactions. Mobile gamers are purposefully conditioned to believe games should be cheap or free and that paying money should be an option. This changeover was made easier by the different demographics of mobile gamers as well. Since the popularization of mobile gaming, demographics have expanded to include significantly more women and young children, many of whom are unaccustomed to PC and console gaming and thus unaware of older pricing structures and marketing concepts.
Here’s another example: On a PC or console, paying $15/$20 US dollars for a quality game is usually considered a deep discount. A deal. On a mobile device, that’s considered absolutely outrageous. Most mobile gamers wouldn’t even consider paying that amount for a game. They’d liken it to highway robbery, even for a high quality experience. Charging them a full $60/70 for a new game? Unthinkable. That’s the reality of the market.
Devs don’t have the option of ignoring mobile gamers. Depending on sources, mobile gaming makes up more than a third of development, and almost HALF of revenue in 2018/2019 so far. I would love it if that weren’t the case, but it’s an unmistakable trend.