Giving cash back creates wrong incentives and you sign up different audience than if you offer airline miles. Airline miles are in demand for people who travel often (affluent) in comparison to people who wants to get $150 after signing up for cc (poor).
That said there are offers to get cash for signing up. Some time ago First Republic Bank was giving $300 for cc sign up for Google employees (aka rich people).
(1) They work because the bank and airline partners are able to offer you something of high perceived value (a vacation, even in business or first class) for very low marginal cost.
(2) This means that (in my most recent redemption) the bank pays the airline $462 in rewards, that cost the airline $400 to deliver, and provides to you with $2500 in value.
(3) Points are great because people don't consider them "cash" but instead something they don't mind parting with. More of a rebate.
So the economics of it are that the bank collects $0.018 in interchange for every dollar spent on a card, converts that to a point that costs them $0.007 to deliver, which the airline services for $0.006 by disposing of inventory that would otherwise go unsold.
The airline wins again because they've created a secondary market for unsold inventory that they can sell to leisure travelers without undercutting the amount they can sell the same inventory for to business travelers.
It's a very nuanced and successful way of motivating people who have a choice to make the choice you want them to make.
On the other hand, $0.01 in cash back costs the bank $0.01, and then if you redeem for travel the airline gets paid $0.01, and you get $0.01 in value.
But conversely the value of points have to be discounted by the risk of loss of utility or future devaluation.
When the credit card says $200 in cash-back rewards, there's a lot less room to play games. 20,000 miles might buy a ticket today, but only an upgrade tomorrow. And that even assumes the ticket you wanted was available through the program you decided to back.