Because it's not clear what "lifetime" means. In practice what it means is "until the company is sold" or restructured or whatever.

But the implication is that it's for your lifetime.

How does your lifetime license save you money over just engaging in a straight-up purchase?

It's very clear what it means, it means the lifetime of how long you think this product is going to last. It's a decision as a consumer you make.

When I purchased the Plex lifetime license 6 years ago, I asked myself how many years to break even vs if I purchased yearly. The answer was 2.5 years.

I also asked myself if Plex was going to last at least that long, and I thought they were, and they did, and I wanted to support them.

FWIW, I regret not buying a lifetime Plex license. I am still paying every year.