Many vendors have bad security practices and ship all their devices with an identical default password (which is easier than programming and labeling each device with a unique password or mandating a password change before it can be used).
When such devices are accessible online it becomes trivial to find them and use such default credentials to abuse them at scale.
The fact that you make effort to change the default password is already sufficient to thwart a large part of that potential abuse, almost regardless of the actual strength of the password you select.
Is it okay to come up with one very secure password to use on all my devices?
Re-using the same password in many places is universally a bad idea.
The main problem is that good security is hard and you can't really tell from the outside if your really good password will be properly secured or not, at least not until the moment that it becomes clear that the security failed, or there was never any security in the first place.
For instance even the best password in the world is useless if the device/application/website will effectively hand over that password, in clear text, when asked correctly. It would be especially bad if that password can then subsequently be used to unlock many more devices/applications/sites/secrets.
If you don't have a password manager already and don't want one either simply labeling devices with a unique password is quite effective and secure against digital attacks, as is an old fashioned notebook.