You, a single person, possible a collection of a hundred or so similar people, are
1. Asking dozens of giant corporations to provide legal licenses to use their software on your system, in a way they haven't accounted for.
2. Expecting them to take the time out of their busy schedule to write over a license agreement, a copy of the software (which they don't have, unless they have copied the ROM themselves), and expect you to use the software in accordance with the license agreement (which if you ever sell or give away the arcade machine, would be in violation of the software license)
Understand that aside from the possible liabilities of releasing such software to parties which may not have the best interest of the companies at heart, what can the companies expect in return? You're looking a few hundred dollars for a few lawyers to come up with the agreement for each game, and then another few hundred for the software development time to make sure these "legal" roms aren't bundled in with all the other illegal software currently floating around the internet.
"More trouble than its worth" applies to serving a niche market just as well as it applies to serving a lawsuit against someone emulating.