In this case, you are assuming every time users open the app, they look for the Action.

From my experience, if a user opens the app intending to search, he/she is ok to take more effort to tap on the search button because it delivers value for him.

But if a user opens the app just for browsing, the CTA will get in the way. Moreover, when he slides up to browse, there will be many chances to tap on the CTA, which's annoying.

Besides, users use their phone in variable ways for different tasks, not always one hand.

This is very useful series about how people use their phone:

https://www.uxmatters.com/mt/archives/2017/03/design-for-fingers-touch-and-people-part-1.php

https://www.uxmatters.com/mt/archives/2017/05/design-for-fingers-touch-and-people-part-2.php

https://www.uxmatters.com/mt/archives/2017/07/design-for-fingers-touch-and-people-part-3.php