I believe the profit margins for the airlines in selling their miles to AMEX, Chase, etc. are quite high (by airline business standards). One of the ways they can drive value for these cardholders and thus continue to sell all of these miles is by having lounge spaces. Also, card issuers need to drive value for their cards and one easy way to provide perceived value.
For reference, in 2023, United earned some $2.7B from the sale of miles to its partners, primarily credit card issuers. Over the same period, they made only just about $1B in profit as a result of actually flying planes, but sales of MileagePlus miles account for ~5% of all of United's revenue.
JetBlue on the other hand earned some $422M from sale of TrueBlue points, about 4.3% of revenue.
Lounge space it for airlines to pamper their most valued customers. Frequent (usually business) travelers who often pay full fare, buy upgrades, etc. These are the travelers who actually keep airlines in business.
These customers are valuable enough that airports risk losing flights (and even carriers) if they don't make the space available to the airlines.
That said, I don't think the credit card agreements are why JetBlue is building lounges - lounges aren't targeted at your run of the mill business travelers either, since they're very high cost amenities. It's why Chase, Amex only offer it to the most expensive cards - it's part of the lifestyle marketing. United and the rest of the US carriers don't include lounge access for most domestic trips, even for first class or holders of their tippiest top status levels. Instead, they're targeted at those who contribute the most heavily to airline revenue - the folks who know they're going to be spending 2 hours in Newark every week this year and are willing to pay the 650 for membership, or are flying international (which tend to be higher margin trips) often, and
therefore have status. United only includes lounge access for mid tier and up status holders on international trips. It's also why the big three differentiate with premium lounges, above their standard club offering. United Polaris, AA flagship, Delta One lounges - the premium lounges are for those who contribute the most most- people who are flying in international business class. They get the most real estate, both in the air and on the ground, because they're paying an order of magnitude more.
My thesis has consistently been that JetBlue doesn't want to be a low cost carrier - they want to be seen as a viable competitor to the big three, which was behind their European push. As part of being viable competition, they need to make the Mint experience also competitive. While it's by far the best domestic first experience, it's actually lacking on the international level, and lounges are a piece of that. Look at terminal E - Air France, BA and Virgin Atlantic, Lufthansa, Emirates etc each maintain lounges there for a scant handful of daily fights. It's because lounges are such a part of the expected international premium travel market. My opinion is, since JetBlue's only international hopes are out of BOS/JFK (since their planes can't do much else), it's them reaching to get to the same playing field as the international majors.