Because the world is complicated and every human activity is financed somehow. Those activities have a variety of risks, time horizons, regulations, participants, etc. A big enough world will generate a lot of ways to cater to those considerations.
step 1: accumulate a buffer of 3-6 months expenses in your checking account
step 2: a) identify money that you will not need in the near future. b) accept that you will not beat professionals at picking individual stocks. c) use the money to invest broadly in the market.
step 3: in preparation for retirement (or a large, time-sensitive purchase), shift allocation to bonds.
if steps 2 and 3 are too complicated, buy target date funds instead. the hardest part is figuring out how much money you can safely tie up in investments, but no school can tell you this. step 1 is also pretty hard if you don't make much money, but that isn't really related to finance.
note that 2b) means you sidestep all the stuff that's actually complicated, and you're better off for it.
my point is that, once you accept that you can't beat the professionals, there is a very limited set of investments that make sense for most people. you can get optimal returns without understanding much about how the instruments actually work.
there are some hard decisions to make along the way, but they depend more on self-knowledge (what's the minimal lifestyle I can accept? how much risk can I tolerate?) than a technical understanding of investment vehicles.
