They can set whatever price they want. Most customers have no choice but to pay; there is no competitor with anything approaching full compatibility or a similar feature set.

Companies like Microsoft and Adobe have maintained a business software monoculture for decades. Nobody has invested significant resources into competing products, just tiny companies and open source volunteers putting out niche alternatives. Microsoft could probably double their prices, and double the built-in advertising, and most customers would complain loudly and keep using them. Docx files, PSDs, PDF forms, etc with any complexity will only ever run properly in one corresponding proprietary application.