4. When their quota/sales target is not met, developers/publishers are under pressure to make up the difference.
5. One of the easiest ways to boost sales is to introduce items which will confer a greatly desired benefit on its purchasers. OTOH, non-buyers who cannot enjoy the greatly desired benefit will endure a comparatively degraded playing experience.
These two aren't necessarily true.
4. Smaller developers especially may not have things like quotas and sales targets which dictate their entire behavior. They're also more likely to be developing for fun as much as income such that $$ aren't the only consideration. And they're more likely to pick free to play as a model just because nobody will pay up front for a game/company they've never heard of.
5. There are plenty of ways around this. Many games don't even require direct player-to-player competition. You can also segregate players so that payers and non-payers can compete in different tiers, or allow modes of gameplay which exclude or dampen the benefits of "pay to win" items. You can even allow ways for non-playing players to gain the same benefits, but in ways that are inconvenient enough the really dedicated will do it, while some others will decide they'd rather pay than put in the effort.