Although this question might be neither serious nor specific, I would write a bit for it.
Kanji pronunciations predominantly consist of 音読み and 訓読み. Everyone who may concern this question should already know this. And here we go:
音読み pronunciations may be adopted from different Chinese dialects in different stages. For example, in recent decades, cities which are related China are usually written in kanji to align to Mandarin orthography and the pronunciations are usually borrowed from Mandarin or native dialect of the city. Examples include 北京(ぺきん), 上海(しゃんはい), 香港(ほんこん), and 廈門(あもい). In other cases, by no means will 京 be pronounced as きん or will 香 be pronounced ほん. Like this time, as Chinese dialects emerged and died, Japanese people borrowed different pronunciations of kanji from the dialects. I will attach a longer list below.
And the 訓読み part is very clear, too. A kanji was used to represent a native concept in writing, like they borrowed 海 to represent the sea which they called うみ, and used 海 in combination with 苔 to represent sea weed which they called のり. They borrowed the meaning without pronunciation for basic concepts which were already developed before adopting kanji. Examples include 緑 and 水. (However, for sophisticated concepts like society (社会) and diversity (多様性), they had to borrow existing Chinese words or invent new Chinese words.) Keep in mind that kanji was the first (but non-native) writing system used by Japanese people and kana were derived from kanji; don't mix the order.
In short: Blame the Chinese.
北京 ぺきん / 上海 しゃんはい / 広州 こうしゅう / 深圳 しんせん / 天津 てんしん / 香港 ほんこん / 澳門 まかお / 台北 たいぺい / 武漢 ぶかん / 福州 ふくしゅう / 廈門 あもい / 東莞 とんがん / 珠海 しゅかい / 南京 なんきん / 連云港 れんうんこん / 大連 たいれん / 瀋陽 しんよう / 長春 ちょうしゅん / 哈爾濱 はるびん / 斉々哈爾 ちちはる / 拉薩 らさ / 烏魯木斉 うるむち