Simply said the answer is because he deserves it (even if he might not need these prayers):
Sending prayers and blessings to the Prophet () is a honorific act and a way to pay respect to him! So it means honoring him, as Allah honored him out of all HIS Messengers, Prophets, Angels and all HIS Creation by the known verse from surat al-Ahzab (33-56).
This can also be concluded for example from the wording of this hadith from sunan an-Nasa-i:
The Messenger of Allah (ﷺ) came one day with a joyful expression on his face. He said: "Jibril came to me and said: 'Will it not please you, O Muhammad, (to know) that no one of your Ummah will send salah upon you but I will send salah upon him tenfold, and no one will send salams upon you but I will send salams upon him tenfold?
(See also a second version in sunan an-Nasa-i)
"Will not please..." comes here with the meaning "isn't that enough honoring to ask the believers to..." or "Isn't that high esteem or respect to ask the believers to ..."
And of course it is a mandatory act which must be performed by each Muslim at least once in lifetime (so far there's a consensus of scholars according imam al-Qurtobi in his tafsir) based on the order of Allah in surat al-Ahzab (33-56) and many ahadith on this matter.
See also: What does "السلام عليكم أيها النبي" actually mean?
Why do we use different phrases for different prophets??
Why so careful about honorifics?
Why do Muslims add "peace be upon him" after names of important people?
and many other relevant posts which you may find at the right of this site.