That’s pretty interesting. Why can’t you do an AND gate just by putting two transistors in series between your input and output?

That's pass-transistor logic, which is used in lots of processors. The problem is that your voltage levels drop through each gate, so you can't chain many gates together. You need to have "normal" gates in between.

A "normal" gate amplifies and restores the voltage level so what you get out is better than what you put in. But in most technologies, the amplification stage also inverts, so you end up with basic gates such as NAND/NOR/AND-OR-INVERT, rather than AND/OR.

https://en.wikipedia.org/wiki/Pass_transistor_logic

Makes sense, thanks for the explanation!