Esmi asked me to step in and try to clear up the confusion between .com and .org as it's being discussed in this thread, so here goes.
WordPress.com is a property of Automattic. Automattic donates about 20% of its total employee time/resources to the open source project. This means people like WordPress lead developers Ryan Boren and Andrew Ozz, committer Daryl Koopersmith, and I are all donated full time to .org. Additional people from wordpress.com teams work on various patches, plugin repo management, themes, what have you. Matt's investment company, Audrey, also pays several people to work on WordPress.org. That's Nacin, Otto, and Scott. Between this small group of paid-by-Matt people, a big chunk of work gets done. But WordPress.com is not in any way responsible for WordPress.org. Donating a group of people is just a choice Automattic makes, not an obligation. There are no "secret paid volunteers" -- everyone who is paid by Automattic to work on core is more than happy to claim that status since it is hard to come by, being representative of being among the best in the field.
Other companies that are in the WordPress ecosystem have (for the most part) not yet taken up the sense of responsibility to the platform that would lead them to contribute resources. There are a couple of exceptions. 10Up donates some of Helen's time (core code contributor), as does Dreamhost with Mike S (same). If every business of, say, five or more employees started donating 20% of their employee time to the project, we could probably move faster. If every independent consultant donated 20% of their time, we could probably move faster. When Automattic was that size, every employee was contributing to WP. Saying "write a check" is not helpful, because Automattic would hire more people if it could find the talent. The level of expertise needed for some of this work, as Esmi mentioned, is rather high. Lots of highly qualified people would prefer to make buckets of money as a consultant or run their own company. Lots of people running companies would rather maximize billable hours and internal productivity rather than investing in the platform. These things means highly qualified people are hard to find. Everyone's looking these days, not just Automattic.
To reiterate Esmi's point: WordPress.org is not a company, and cannot hire people, as there is no legal or financial structure to do so. The WordPress Foundation is an educational charity, not a trade association, so it can't hire developers either. But hiring them isn't the problem. It's finding people who are good enough and who are available (and/or getting other companies to donate some of their best and brightest the way Automattic does).
Resources is a big issue, yes. But frankly, it's just as big an issue that we have a fairly convoluted infrastructure, and making big changes is a lot more complicated than it looks. The forums, on an outdated standalone bbPress, haven't been updated to the bbPress plugin because of x, y, and z. The WordPress.org site itself runs on flat files rather than WordPress, in large part. There are a lot of infrastructure issues that need to be solved for meaningful change in these areas, and there is a group of people working on ways to update these systems. Decisions are made every day about which bits to update and improve next. Forum search (ditto Codex search) comes up a lot. Using old versions of bbPress and MediaWiki makes these two parts of the site harder to deal with than some of the others.
If Matt - and let's face it, he does have the BBQ money - hires a bunch of people tomorrow to sit and secretly work on wp core as "voluntary" contributors to the wordpress code - that would solve the problem
It's not a matter of money, it's complexity. If you can find "a bunch of people" that are actually qualified, send 'em over to apply at Automattic, Audrey, or other WP-based companies that have committed to contributing to the project. But they need to be really good if they're going to get the keys to WordPress.org.
I do not understand why the people who make huge bucks off wp don't invest in core development.
You and me both, my friend.