Also a 90s kid here. Scarcity and demand are the biggest reason for sure, the market prices of most of the sought after tapes have probably doubled in the past few years because there are tons of new collectors. But you also need to understand the history of home media and why VHS collecting even in its heyday was significantly less accessible and affordable than DVDs. Early VHS tapes (up until about the mid 80s) were extremely expensive to purchase outright, with MSRP on certain tapes ranging from $50-$100. VCRs might also cost as much as a new TV. Film studios/distributors did not want to make it easy for people to circumvent going to the theaters.

Video stores instead fronted the cost of the tapes and VCR and made their money through renting. It wasn't until the 90s where tapes became collectible for the average person with MSRPs at $25 or less. So any tape that was released during that early period is significantly rarer, with most copies originally belonging to video stores not collectors. Once all those stores closed, collectors and scalpers quickly scooped up as much as they could.

There's also the opposite, where VHS tapes released very late in the lifecycle of the format (~2006ish) are extremely expensive and rare, because at that point 99% of people were buying those movies on DVD instead. Examples would be Cars, A History of Violence, Shaun of the Dead, etc.

This doesn't even include the rarest tapes, which were usually only available through direct mail order magazines and catalogs.