a) Implication that it's complex to add compression.
Other common filesystems have it, NTFS, ZFS, BTRFS. Given the amount of money Apple have, I doubt it's that complex.
b) Disks are cheap.
Looking at apple.com/uk pricing, the difference between the 512GB SSD and 1TB SSD is £400. And if you choose incorrectly, you can't just open the case up and change it.
You could then use an external USB, as I do, but sleep/wake doesn't work properly, and endurance on USB keys isn't exactly ideal.
Or you could hang a whole powered drive off it, but then why have a laptop.
c) Nobody needs compression
I've got a couple of hundred GB of virtual machines. I save about 1/3 of the space by just compressing/uncompressing them all the time, but it's a waste.
I imagine a lot of developers would also find it useful; I'm playing around with 70GB text file I generated as part of some stuff I'm doing, and that compresses down to about 5GB when on my server on ZFS, but then I can't carry it with me (e.g. use on a train).
I could spin up a VM with ZFS in it, but then I'd need to hard-allocate a bunch of space to a fileserver... and back to square one.
You may have a different bunch of concerns. SSDs might be plenty big enough for you.