Just to add more to the discussion, but it doesn't seem to me that most IP's are even "on the market". Most owners are the companies who produced them, the developers themselves or the companies which bought out the rightsholders as a whole, usually a bigger studio. It's not like some guy off the street can make an offer and just buy these games most of the time. Furthermore, even when some of these games get released, it's through a distribution deal, where the base rights aren't actually transferred, just the right to sell them. Often some percentage of profits still funnel back to the base rightsholder(s).
And it gets more complicated than that...
Very often multiple parties have rights to a given game. One company may own the IP trademark while another owns the game itself. Sometimes the game itself is partly owned by two entities, the main company and the studio hired to actually produce the game. Sometimes music deals were made and the artists have some control of the music. Maybe it was a time-limited deal or maybe it covers a particular distribution form and a digital release would require a new agreement. To get the distribution rights, one has to get all the rightsholders to agree AND the deal has to be good enough that you still think its profitable.
Now, we haven't even gotten into the legal mess of corporate buyouts, where not even the companies involved know exactly who owns what as a company goes bankrupt and multiple suitors cut up the pie. Remember that with classic games, while we are talking computer games, digital documents were not yet a thing. So most of these agreements sit on pieces of paper somewhere or lost. And in their absence there is a legitimate debate on who actually owns the rights. Then add that most of these agreements involved physical distribution. The language in the agreements didn't anticipate anyone would want classic games years into the future. In this absence, maybe arrangements have to be renegotiated and some people are no longer with us, so such negotiation lies with the inheritors if those can even be found...
And at the end of it all, most of the time, the actual owners, since someone else sees it as potentially profitable, want to keep the base rights and make a little long-term profits themselves.
In short, while its easy to say, "buy up more games", the reality is that its far more difficult than that.