DCAP basically is FSA spending not on yourself. That is pre-tax dollars.

The Child/Dependent Care Credit is a Credit, so it gives a better dollar-for-dollar impact.

And you have to examine income and qualifications and number of children/dependents, to answer why one or the other but not both. And yes, filing status matters; married filing joint is required for the credit.

I found some resources for you:

https://turbotax.intuit.com/tax-tips/family/the-ins-and-outs-of-the-child-and-dependent-care-tax-cre...

https://www.benefitresource.com/blog/dependent-care-fsa-or-child-and-dependent-care-tax-credit/

"and why do they allow MFS spouses to contribute when they cannot benefit?"

There is benefit; you just need to use the better option, depending on facts and circumstances.