You gave an example that works and one that doesn't. Apple certainly is very capable of doing what you want but chose not to do it. There must be a reason - I bet it's the adding complexity. They just wanted to make a simple to use mail app that even grandpa can use. They did provide the mechanisms in the OS for anyone to implement a complex version, and you'll find them in the app store. Those apps you've mentioned can send files as email attachments; whether they implemented sending more than one attachment is up to them. Within the Apple implemented apps, the philosophy of sharing files is very simple and narrow. Take it as it is but do not say it can't be done. Let's take as an example of the Photos app. You can select several photos inside the app and send it by mail as attachments. It's just a different way of doing things that not everybody likes. You can't make everybody happy - specially when you're trying to make things very easy to use.
I understand and completely respect your arguments. The challenge is that while Apple is making that mail app that appeals to Grandpa, the rest of the marketplace is becoming more savvy about what they can do with their devices and their opinion about what they "should be able to do" with the stock software is changing as well. Apple is looking like a device for Grandpa while everyone else buys Android because it is easier to do the more "complex" tasks (although I hardly think of attachments as complex).
If Apple wants to take the philosophy that mail "starts" from somewhere else, that is fine. But for a huge part of the market mail "starts" in the mail app.
Perhaps it is better to use a Mac for these kinds of tasks and the iPad should be reserved for more casual use - however, with competition providing an alternative that seems "more useful" Apple is really suffering.