Why is there no 'Tap Tempo' feature

Hope this helps:

GarageBand for Mac: Set the tempo

You can set the basic project tempo in the LCD. The tempo is displayed in beats per minute (bpm), ranging from 5 to 990 bpm. You can also tap the tempo using your computer keyboard. When you add Apple Loops to your project, they match the project tempo.

Set the project tempo in the LCD

• Click the triangle on the right side of the LCD and choose Beats & Project, then do one of the following:
• Click-hold the tempo value and drag up or down.
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• Double-click the tempo value, then enter a new value in the Tempo field.
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You can also add tempo changes using the Tempo track. For information about the Tempo track, see Tempo track overview.

Are you looking for a click track, so you can sync the tempo of an audio file automatically with your playing? GarageBand does not have a tool to detect the tempo automatically like you can do in Logic Pro X or other professional software.

I am occasionally faking a click track by recording myself, while I am clapping along to an audio recording. And then I am adjusting manually the automation curve for the tempo track, so the measures are in sync with the clapping.

I think I misunderstood your question. On a Mac the "Tap tempo" button is available, when you create a new project. You have to click the "details" menu, when you select the project template.

That is only part of GB for iPad. Besides, what would you tap?

It is so hidden away, that I completely forgot it is there! I have never used it.