Because it sells consoles? It happens with new consoles aswell, before they release their is an E3 with promises and games that will come but we don't know when so we see people rushing to get consoles (and occassionally bitch a lot first year because their are not that many games).
For some games I forgive them, announcing Samus returns without announcing MP4 would make people mad at Nintendo. Bethesda every year has to answer why ESVI is not announced and they have stated many years that the vision they have with ESVI is not possible with the tech that is out today.
E3 is also for investors, stock from nintendo went down after their conference. If they announced some games that will not be available for years it probably would went up.