That defeats the purpose of a fingerprint sensor.
No, it does not. Most people don't reboot their computer daily. Mine has an uptime of currently over 20 days, likely because that's when the last MacOS update came out that required a reboot. It will still let you unlock with biometrics after locking the screen.
There are two reasons for this behaviour, the first one is that asking for the code from time to time ensures that users don't forget it. I have seen a few situations where people kept using biometrics to unlock and ended up forgetting their code altogether, and lost access to their data when biometrics ended up not working. For this, the OS forces you to enter the code at least once every 7 days, even if no reboot or logout has occured. I too am annoyed by this, but I have forgotten PIN codes in the past and after a couple weeks who knows if I'd still remember it.
The 2nd reason is that the biometric data is actually locked away by the code. So after a reboot your Apple device has no access to the biometric data, if you press your finger on the scanner it can scan your finger, but it has nothing to compare it to. It is unlocked when entering your code for the first time after rebooting.
And it will stay unlocked until one of three conditions are met: You reboot the device again, you log the user out, or the mentioned 7 day timer runs out.
I don't see how entering a password is more secure then biometrics
It is. Back in 2013 TouchID was tricked by researchers who took high resolution images from fingerprints found on the housing of the very same device (an iPhone 5S) and were able to manufacture a high resolution replica that unlocked the device with TouchID successfully. That is a very basic attack that Apple can't protect against, since your device has no way of knowing whether the finger used is actually yours, attached to your own body.
To put differently, biometrics are something you are, whereas a password is something you have. Those are two fundamentally different factors, and they are treated differently by law as well. For example, in the States police have in the past used biometrics to unlock the iPhone belonging to a deceased suspect. And depending on your local laws you might be required to unlock your device with biometrics but not with a password.
In some situations biometrics are more secure, in some situations passwords are better. There is no clear winner, but the technical implementation by Apple makes it impossible to use biometrics after a device reboot, and these functions are based in the proprietary firmware and soldered on to the Apple device's logic boards, there is nothing that can be changed about it.
...which is a good thing. I wouldn't want some workaround to circumventing device security preventing a third party from accessing my personal data.
