PostmarketOS is the mainline Linux kernel and doesn't have all the features that are needed for a proper Android phone. There are a lot of Android userspace drivers that may break with another kernel.
Custom kernels are not upgraded kernels. They usually just back port a few fixes. No custom kernel running Android has a full kernel upgrade because it breaks the KMI and kernel drivers aren't usually updated.
That's not entirely true. There's plenty that bundle a custom kernel.
Browse some device-specific development topics on XDA developers for examples.