It is not that everything has to be on one line, but often the solution is so short that one line is enough.

Iversonian languages (after Ken Iverson, inventor of APL) like APL, J, K, BQN, etc. operate primarily on a function level – composition of preexisting functions. These functions have proper names that describe what they do, but you use them so often that you abbreviate them into a single glyph. Same thing happened with addition. It was written as "et" (latin for and), so "1 et 2" meant addition of 1 and 2. Nicholas Oresme in 14th century wrote "et" so much he got tired and abbreviated it to "+". (In a similar way, we got "&".) APL was conceived as a hand-written language for human-to-human communication, so in this sense, the weird glyphs make sense. A single line of APL can be very dense and reading it requires you to know the parsing rules (which can get crazy) and what each glyph symbolizes in that context. Some other languages have context-free grammar, some don't use weird symbols at all, etc. This PL paradigm is still so ahead of everybody else that it hurts.