It's possible, but nobody does this because the friction to link an account with plaid is very high. You need to pick your bank then give your username and password to your bank (and potentially a 2fa code etc). All this just to get an ACH and routing number.
Plaid's true value add is normalized access to transactions and other banking metadata. Using it only for payments is not a good UX.
I work at Plaid and I agree that using Plaid just for getting an ACH and routing number leaves a lot of value on the table. You're getting more than an ACH number and and routing number though -- you're also getting proof that the person can log in to the bank account associated with that account/routing number combo, which is really helpful for fraud reduction -- fraud in online payments is a huge problem and part of the reason many of our customers use us. (You also get, if you want it, a token that you can use directly with Stripe, Dwolla, etc. to do the funds transfer.) Another popular use case is to use the Plaid Auth API for ACH and routing number, but then also use the Plaid Identity API (again, for fraud reduction) and/or the Balance API (to help make sure that the payment won't bounce).
Plaid's true value add is normalized access to transactions and other banking metadata. Using it only for payments is not a good UX.