Why don’t manually entered activities show up in MyFitness Pal?

Because only Calories measured directly by the Garmin device count. If you want to adjust the Calories manually, you have to do it on the side of MFP.

I do not know what Pickleball is, but with the majority of Garmin devices, you can add any sport not available directly, as a Custom Activity (also often called "Other" in the menu), and name it in the way you like.

But it doesn't generate calories  by adding a non-garmin loaded activity

But it doesn't generate calories  by adding a non-garmin loaded activity

You mean Calories in MFP or in Garmin? In Garmin, you can only change consumed calories from the side of MFP. Burned Caloreis are exclusively calculated by the Garmin device, on the basis of the heart rate. No manual entries, and no transfers from third parties like MFP are allowed.

And pickleball is a very popular sport.  A cross between tennis, ping pong, and badminton

OK. As I told, just add it as a custom acitvity.

So Garmin only calculates calories based upon my Garmin device‘s heart rate, not the activity type that I select?

Correct. It only uses the current heart rate, the average resting heart rate, and the heart rate variability (HRV) to calculate the Calories burned. Steps and motion patterns, or activity types, would be only used if you had a device without a HR monitor.