Line is mostly used in Japan, Signal is mostly used by nerds. So you can assume neither japanese peopel nor nerd were their primary target. Given that this is true for the vast majority of the world population I'd not call this odd.
By contrast, Telegram has a quick selection of the last few pictures available immediately, and the general picker opens quicker if you request it -- maybe because it's using the stock iOS picker rather than the custom one Signal implements?
Since sending a picture to a contact was a super common action of mine, this was incredibly frustrating.
Signal also had notification issues if I was running the desktop client at the same time. It'd sometimes clear notifications from my phone before I'd actually read them in either location.