It's just not the same cheap piece of plastic, that is the answer. First of all, it hasn't been "a couple of years"; the series is eight years old this year with at least one new version or revision almost every year.

and most importantly, each new launch comes with improvements to the screen, buttons, specs, charging port, wristband format, battery, features... along with frequent price bumps (I wager the seven with all its rumored improvements will cost €45-50 at launch).

gadgets getting improved upgraded versions yearly is standard practice in the consumer tech industry, and given that the mi band series collectively sells in the tens of millions of units, and is in the hands of many people, it is no surprise xiaomi would want to keep outdoing itself and satisfying costumers.

K.
Any particular reason to have 7 versions of the exact. same. cheap piece of plastic - in just a couple years?
Gosh, it reminds me of those vaguely racist anecdotes ("how many people does it take to screw in a lightbulb...")