I had to はし (hashi) over the はし because I forgot my はし at home.

Same word phonetically, three meanings. With Kanji it's easy.

We also use two different alphabets. Lower case and upper case. Upper case is basically Latin script optimised for stone carving, lower case was developed for ink writing (I think in the Carolingian era). Now we use both at the same time without batting an eye.

Add cursive in the mix and we also have 78 letters instead of 26.

Nah, it makes sense. You can write everything with just hiragana if you want to, in theory.

Katakana denote words from different languages, which I found really helpful when learning the language. It's probably easier for fluent people too. There are a lot of these words.

Kanji are a lot more compact and make text a lot more readable. Japanese does not use any whitespaces so it can be tricky to separate words when using only hiragana. Instead you mostly have some kanji separated by hiragana. Some Kanji only denote a single hiragana, but usually they represent a group of them therefore saving on space too. Like other languages they have words with multiple meanings, but they have different kanji, further improving readability.

Take this with a small grain of salt, I'm by no means fluent myself, but I've been learning for quite some time.

As a Japanese learner, katakana is a godsend. It's like reading a scientific paper in English and having all the Latin in italics, as an indicator that "don't worry this is a foreign word, you're not an idiot for not recognizing it." Especially because most katakana words are derived from English (or words you'd recognize as an English speaker) so it's just a matter of saying it over and over until the pieces click into place. Example: オーストラリア = Oosutoraria = Oh-s-t-rah-ree-uh = Australia.

Also outside of picture books for young children, Japanese doesn't use spaces and has way fewer sounds than most languages which results in a LOT of homonyms and similar words that all blends together (see other comment YouTube link). So having three writing systems in one really helps convey meaning and makes reading much faster.

Why doesn't English fix its spelling?"

Because this is what Japanese would look like of they didn't use kanji.

To be fair we have two alphabets upper case and lower case. The hiragana and katakana are basically the same. One to one equivalency between them. The kanji does add a lot of complexity.

No it adds variety to the look of writing, each character is a syllable not a single sound (mostly) so they use fewer characters for per syllable, having two syllabary systems means that there's more visual distinctness per word. I'm not a Japanese speaker so don't take my word for it, but no they're very much not stupid it's a clever system and one that's related to the history and culture of Japan.

Modern Japanese is a chimera of native words, Chinese, Pali, and various European languages. Kanji are used to write the Chinese loanwords, hiregana for the indigenous stuff, and katakana and Romaji for the European loanwords (sort of). You could write everything in hiregana, or even in katakana or Romaji with some effort, but doing it this way is easier.

I know it's a rhetorical question, but It's a result of the popularization of ideograms during the spread of writing technology in their region, as opposed to the representation of concepts through only patterns of a small set of character seen in Europe which later spread to the far west. They're far from the only culture to make the choice.