Its not a lack of money per se that usually causes these things to be missed, its more a lack of personnel and/or time.

Companies often find just the right number and mix of people to get jobs done. Yes they could hire more people to implement those extra features but then those people would need to be trained, granted access to the source code, talked to around the water-cooler; and any of those things, together or separate, no matter how mundane, have an impact on the company that could actually cause less to get done despite having more people.

Theres a size-time-money dynamic where each of those variables affects the other two. Just the right mix needs to be reached.