Why doesn't the Tennis app count steps?

Seems like it should I agree but it doesn’t.  Does the aerobic activity?  You could try that instead.

I can see where it wouldn't  Your wrist is what "sees" steps, and when playing tennis, the watch doesn't know what are steps and what might just be arm movements.  Garmin devices also have logic for step rejection, where it can reject moments are probably aren't steps.  At one time, if you took less than 10 steps and then stopped, they were ignored.

I did some comparisons between different brands of trackers a few years back, and one brand in particular WAY over counted steps, but people didn't complain, as it was easier for them to make a step goal.

I played badminton and got only a few steps.  That said, it really doesn’t matter.  You still did the exercise whether or not the watch notes the steps.   Reaching and arbitrary step goal isn’t really important. 

I wear my watch on my left wrist and hold my racquet with my right. I use the tennis activity for playing squash as that seems to be the nearest genuine Garmin app available. Others seem to do a reasonable job of tracking steps during racquet sports.

I'm not so bothered about any arbitrary step goal. But no of steps seems to be a useful metric to compare how intensive my games are. 

The watch itself does count steps - I usually have +4-5K for one hour session. The activity details include distance both for open air and indoor tennis too. If you need to see the number of steps specific to the activity, try installing a "running"  or "walking" datafield that does that for one of your screens. E.g., both Dozen Run and Dozen Walk show number of steps in the activity stats uner "Connect IQ" field. 

Re the intensity of the games - steps are irrelevant here. I usually judge intensity by my HR - average and peak. It's also reflected in the number of Intensity minutes added during activity - they are disclosed in the activity details.