Why does the pianos register include more than 7 octaves?

In fact, the reason that the piano has 88 keys (or more than 7 octaves) is because that's the range that was finalized by the American Steinway piano makers in the early 20th Century. Until then, the piano's register had been growing constantly from the original 5 octaves (49 keys, F through F) of Mozart's pianos to 5.5 octaves (56 keys, F to high C) in Beethoven's required register, to 6 octaves (61 keys, F to high F) which were called for by Schubert, to 7 octaves (85 keys, A through high A), which is what most composers of the greatest piano music would have been content with. The Steinway manufacturers figured the highest note on the piano ought to be C rather than A, so they extended the range with three more keys in the upper register. It was there that the range of the modern piano would be finalized. Older pianos with 85 keys are still in existence though, especially in Europe. The Bösendorfer piano makers still make their "Imperial Grand" pianos with 100 keys, extending the range to over 8 octaves, from low C to D above that of the modern Steinway. Few composers have ever actually written for such a register, and as the extra notes are simply not popular enough to merit their being included in the standard piano, they remain only on the Bösendorfer Imperial Grand.