Because click fraud means higher customer acquisition cost for advertisers, which means advertisers stop advertising.
It also drains advertising dollars away from legitimate content producers to fraudsters meaning that they switch away to other ad platforms.
To maintain a two-sided market you need to ensure both sides of the market are healthy, click-fraud damages both sides of the market and so is very dangerous.
Which is a nice theory, but there's apparently already a huge amount of click fraud in Facebook advertising and Facebook haven't done anything about it so far.
It also drains advertising dollars away from legitimate content producers to fraudsters meaning that they switch away to other ad platforms.
To maintain a two-sided market you need to ensure both sides of the market are healthy, click-fraud damages both sides of the market and so is very dangerous.