Science typically requires advanced degrees, publications, etc. You're not doing much of anything a lay person would consider you a "scientist" for with just a Bachelor's. Getting these degrees serves as the hurdle/filter.

Math is the same as science. You're probably not getting hired as a mathematician if you just graduated with your Bachelor's.

[Non-software] engineering has all the professional certification, shadowing, etc. that is required to get very far in the field.

Software is the only STEM career you can get into right out of high school, being completely self taught. Yes obviously a degree helps and it's much easier with one, but not having advanced degrees is a non-starter for most STEM careers, let alone not having one at all.