First thing: Privacy. I am aware that iOS is not entirely private too, but I trust Apple Photos much more than Google Photos. You can even enable end-to-end encryption iirc.
Second point is control over my data. I can easily export my photos from Apple Photos as files, whereas Google maliciously separates Photos and Metadata upon export. In my experience this is the same for a lot of other services as well. Being able to easily export my data enables me to escape the walled garden more easily should I get fed up with one system. I also try to use as many open source services as possible for this as well as other reasons.
Apple has a lot of malicious practices too, especially when it comes to EU citizens and third-party app stores, etc. - but in my experience Google is no better.
Lastly, I considered switching to an Android with Graphene OS (privacy focused Android derivate) a couple of times, but the added control over your data comes with a lot of other inconveniences. So for now, I’m just sticking to iOS.
When you re-import the images into another program/library, they will not be displayed in the correct order and all other information will be lost as well.
Metadata in general is very useful and contains a lot of valuable information like location data, lens, focal length and device information which you have to manually re-integrate into each and every photo.
I mean yes, I could write a quick and dirty Python script for this, but why should I have to do this in the first place?
In my subjective opinion this is malicious as in it only being this way to make it as hard as possible to migrate away. I highly doubt this is the way their servers store the images as it is very inefficient and the images are likely stored in a database instead. This means in order to retrieve a file they have to process each image anyway, so why not follow the universally accepted and well defined standard and include the metadata in each file?