It's simple economics.
2023 ave. attendance:
WNBA 6,615
NBA 18,322
2023 ave. ticket price:
WNBA cheapest team - expensive team $24-73
NBA cheapest team - expensive team $138-$611
Reg season games:
WNBA 36
NBA 82
So an NBA team averages 2.28x as many games with 2.77x as many fans with a ticket that is 5.75 to 8.37x as expensive. So very coarsely that is $36.31 to $52.86 of team NBA ticket revenue for every dollar of team WNBA ticket revenue. And that isn't even counting the difference in ancillary revenue such as TV revenue, sponsorships, merch sales and F&B. Keep in mind that empty seats don't buy beer, hot dogs or popcorn.
I'm pretty sure there are some out there that would take money from Lebron James and give it to WNBA players if they could. Can't pay players money you don't have without further subsidization.