Firstly, I would recommend researching your question before posting a question that has already been answered and asked by other people. If you don't find your question asked and/or answered, only then is it best for you to ask.
@UmH has given you a link that will answer your question. See here
Secondly, I think the question you should be asking is, 'why does Allah use "WE" instead of "I"?' But, I will attempt to answer the question given.
Thirdly, to the question: let's address WHY "He" is used to refer to Allah.
Allah has no gender. Gender only exists for species that reproduce. Allah wasn't born and He doesn't give birth. In the Arabic language, there is no word for “it”. Even if there was a word for it, it wouldn't be respectful to call anyone “it”, especially Allah.
In the Arabic language, the default gender (if not known) is the masculine gender. Hence, Allah chose masculine pronouns for Himself. Using a feminine gender (she) would have been a real problem since it indicates the female gender in Arabic as well as English.
Refer also to: here
Take Note: The usage of both plural and singular pronouns referring to Allah (SWT) are seen only in the first person such as “I, my, we, our”.
You may also come to notice that some verses of the Qur'an also use the context of "we" which, by the laws of English means more than one person. However, that's not the case. "We" doesn’t come to mean plural in the verse of the Qur'an. It is used as “royal we” for which many languages including Arabic, English, Hebrew, and Persian provide the usage. The “royal we” is used for the singular person displaying the meanings of royalty and grandness. So, “we” used to refer to Allah in the verses, doesn’t indicate the plural of the noun. Rather it displays the majesty, grandness, the endless power of Allah, and sometimes anger and wrath for the disbelievers.
There are verses where Allah refers to Himself as "I". For instance:
"And when My servants ask you, [O Muhammad], concerning Me - indeed I
am near. I respond to the invocation of the supplicant when he calls
upon Me. So let them respond to Me [by obedience] and believe in Me
that they may be [rightly] guided."
Qur'an 2:186
TL;DR:
In essence, Allah uses "He" because it's gender-neutral and specific. It's also used to differentiate Himself since He is not a person, but a diety. "We" is used because it's a way for Him to emphasize His grandness and royalty, which is why the royal "we" is used. There is no such thing as a royal "I". He only uses "I" when He needs to be in first-person singular, if He was giving an order to someone (if the subject itself contains a verb). (see the Qur'an quoted above, notice that He is giving an order towards Muhammad (saws). Hence, He uses I to refer to Himself). The rules of the Arabic language don't correlate with the rules of how English generally works out.
