The problem is that the intention must be recognized by machine, not a human. And : is used quite often for other (also very common) purposes. So the parser would have to distinguish somehow:
word: my description
e.g. from
Johny said: "Hello to everybody." (where different or no quotes at all could be used)
or from:
The train arrives at 20:00.
or from
Fill here your name: .......................................
or from
Dear Sir:
at the beginning of a letter
and possibly from bazillion other use cases, some in different cultures and languages with different typographic rules.
Similarly, your 2nd example matches the syntax of an ordinary unordered list. And yes, it’s very common to use some preceding paragraph ended with a : to introduce it.
Buy list:
- bread
- milk
- some fruit