Plastic is an organic molecule. Think of it like a complex set of legos, and the individual lego bricks have a tendency to want to stick to other things if they get shaken. At an atomic level, everything is always shaking, all the time. So this shaking energy, over time, will increase the probability that some of those lego bricks are going to fall off and stick to other things or just fly free. There are simply more ways these lego bricks can be arranged in ways that are not plastic than ways they can.

Or another way of looking at it, there are nearly infinite ways you can break or damage a porcelain teacup, but only one configuration where it works as a cup that people can drink out of. IE: The chances of it not being a teacup anymore are greater than the chances of it remaining a teacup over long stretches of time.

What does all this mean? Your tapes are literally falling apart. Even if they’re kept in boxes or on shelves away from other energy sources like light or heat, they are still vibrating, they are still shaking. A few molecules here and there, pop off every few minutes or more, never to return. While it might be centuries before they turn to dust, these changes over time will in fact start to smear or degrade the subtle magnetic alignment in that plastic tape which is what the actual audio and video is encoded as. This may take only a few more years to be unreadable depending on the age and quality of the tape.

If you want to save your collection, invest in something to record them onto a digital medium, and even then the best you can hope for is a few more decades. Currently we don’t have many commercially available methods for long-term data storage.