One of the concern is that it impacts battery life, so people turn it off most of the time unless they plan to go to a public space. Then they forget to turn it on.
It’s possible many of those users did not grant the contact tracing permission on the phone when installing. But I’m assuming anyone who went to both of installing would be open to allowing it. They could also have uninstalled 10 mins later. I don’t know how the number is being calculated.
I’m on iOS and keep app installs on my phone to a minimal but haven’t noticed battery drain so far. Android my differ.
Also your phone can collect the data by enabling in Settings > Privacy > Health > COVID-19 without the app at all.