It all makes sense when you think about the way it will be parsed. I prefer to use newlines instead of semicolons to show the blocks more clearly.

for file in *.txt
do
    cat "$file"
done

The do and done serve as the loop block delimiters. Such as { and } in many other languages. The shell parser couldn't know where stuff starts/ends.

Edit: I agree that the then/fi, do/done case/esac are very inconsistent.

Also to fail early and raise errors on uninitialized variables, I recommend to add this to the beginning of your bash scripts:

set -euo pipefail

Or only this for regular sh scripts:

set -eu

-e: Exit on error

-u: Error on access to undefined variable

-o pipefail: Abort pipeline early if any part of it fails.

There is also -x that can be very useful for debugging as it shows a trace of every command and result as it is executed.