Because you're early into the show. It repeats itself into oblivion not long after, to the point that after around the time Inuyasha unlocks the second additional technique of the Tessaiga, it's more or less a parody of itself. The same thing happens over and over and over with the Tessaiga powering up only for its new power to be made useless by Naraku's next upgrade, and side villains who are against Naraku are just time wasters that lead to Naraku transforming again or gaining more jewel shards, ad nauseum. It's also one of the very few manga over 50 volumes where the main villain doesn't change until the very end of the series. Even Bleach changed out Aizen after the first three to four arcs.