Because multiplication is a shorthand of repeated addition.

2 × 3 is 3 + 3, 3 × 5 would become 5 + 5 + 5, etc.

So 1 + 2 × 3 will give you 1 + (3 + 3). That 1 is just a simple number, it doesn't tell you anything about how many times you're supposed to add 3.