There are a few questions embedded in here:
Why are opt-ins and T&C's not placed inline with form fields?
Because it interrupts user flow, which can result in abandonment. Forms are already cognitively "complicated" to users....they have to navigate from field to field, think of an answer and input it. So a lot of effort goes into making forms "flow" for users, e.g. if the user is filling in contact information, they understand that entering email address then entering a postal address are part of the overall task of entering contact information.
When you interrupt that flow by forcing the user to think about opting into a list between fields, you are forcing them to context-switch, which is a poor idea. You want users to stay in flow because once interrupted, they may just decide to abandon the form altogether.
Why are opt-ins and T&C's placed above the submit button?
First of all, this is not always the case. Sometimes they appear below. But, for the majority of cases they appear above because:
They are form elements, so it's awkward to have a form submit (which usually results in panel dismissal or a page jump) halfway through a form. Users will fill the form, hit submit, and may feel like the form got hijacked before they finished reading top-to-bottom.
Placing the submit button below T&C's and opt-ins has UX value in terms of providing tacit agreement to those terms. While this would not hold up in a court, it still makes sense visually and behaviorally. You want users to agree to your T&C's (or to opt-in) so good design will give the "submit" button the affordance of "I agree to this form". So placing it below the T&C's and opt-ins allows you to take advantage of that affordance.