If an 'Enterprise' applications website were functional people would be able to navigate to assistance when using the app. Therefore costing money in competent support techs or improving the product itself, neither of which are as easy as just being anti-competitive and monopolistic
I was referring to the web applications themselves, rather than the marketing, documentation, or support websites. In large vendors, those tend to come from a different part of the organisation, so are often superior to the products themselves.
The productivity drain of a poor UI is largely felt by the customers' employees, while the vendor benefits from sales of professional services and premium support contracts.