An alternative way to see why that’s the most logical viewpoint is, that, if you assume not all marbles in an empty bag are black, you can have the following:

- Take a bag for which “all marbles in this bag are black” is false

- Add a black marble to that bag

- After that, “all marbles in the bag are black” becomes true

As is often the case in mathematics/logic [1] with these kind of things, you can hold an alternative view where “are all marbles in this bag black” doesn’t make sense for an empty bag or one where it has the answer “yo”, but if you do that, you’ll find that many proofs get more cumbersome because you have to handle that case differently from all other cases.

[1] other examples are the convention that 0⁰ = 1 and the convention that 1 isn’t a prime number (that’s an example where mathematicians (eventually) decided that having 3 kinds of positive integers (prime numbers, composite numbers, and 1) is the better choice)