Because rims/spokes are a ridiculously tiny fraction the total amount of aerodynamic drag. Is it a statistically significant amount of drag? Yes. But it's on the order of 1 or2 digits of watts depending on speed. Read: less than a kph/˝mph at normal speeds.

By all means, go buy carbon rims, if you'd like. Carbon rims are awesome. The added stiffness/responsivness from a deep dish wheel...The acceleration from a decent light wheelset is something that's just cool to behold. But don't expect to set your tires on fire with all that extra speed. Their actual gain, even among the best like ENVE 7.8/ZIPP 808's, etc...in relation to what you already have won't be as much as learning to hold a more aerodynamic body position & training to put down real power & sustain it in that position.

The faster you go the more they become worth the cost. Even more so when the wind is at slight yaw angles. But even then, how much time do you spend above 25mph/40kph? Enough to justify a few thousand dollars cost? Maybe...

New tires for example can save you 30-50 watts or more of rolling resistance depending on where you are starting from. Say, Gatorskin Hardshells with standard tubes & moving to GP5000 & latex or tubeless...so 1 or 2 mph depending on speed.

$50 Clip-on aero bars can easily net you 2 mph, depending on speed.

Strength & stamina workouts on a trainer can be another 2, 3, or 5 mph or more before the start of a season & costs nothing & will net a lot more ability than a carbon wheelset.

I'm writing all this as a guy that owns far too many ridiculously expensive carbon wheelsets. Rims aren't the reason some people are fast...It's the strength/stamina threshold, V02 max workouts 4 days a week.