Back in the 90s, people just had no concept of today's media offerings. Content was edited specifically to work with the only home media they new of at the time, and that was interlaced TV at frame rates of 29.97 or 25. There was no concept for progressive displays. The only home video format that was in wide use was VHS, and TV shows just didn't find their way there. That was something for theatrical releases. TV shows were much more concerned about trying to make it to syndication. When it came time to licensing, that's all the producers had on their radar.

What's the point? This was much less a malicious thing than it is made out to be. Once the licensing ran out, that's it. They can't just YOLO their way through it, or they'd have been sued. It's possible they tried to negotiate new terms for the music, but terms couldn't be agreed. When it came time to release on DVD, the person involved for the music might not have been available or interested in doing it again. At that point, the music would never feel right when replaced. The last point being these producers would be doing this on the cheap, so your <$30 CD library wouldn't have been far off, except the music libraries would have been much more expensive than that. Decent royalty free music has only been a thing within the past 10-15 years.

Edit: one more thing about the music, it is a large expense for the production. the studios are usually willing to pay for it to air, because they know how much ad sales they have and build it into part of the per episode expense. negotiating for DVD release with no known amount of money to earn makes it difficult to negotiate a license for "real" music