The send button is a button that confirms your intentions to send, not a confirmation dialog.
The most routine action one does in Signal is send text messages. Should the message send automatically as soon as the person stops typing for a second? I don’t understand why needing to press a send button when a message, be it text, a sticker, a picture, a file, whatever, reduces the “spontaneity” of the message (or why spontaneity is a particular value that Signal should be encouraging). Ease of communication, rather, is what’s important, and having a consistent UI helps much more than sending some things automatically when a user doesn’t know they will send or doesn’t want to send them yet.
I’ve mentioned the need for a consistent UI for sending elsewhere, but this is especially important in this case since it makes no sense that sending one type of picture (a sticker) is handled differently than sending other types (a photo, a gif, etc.). Well, since stickers are associated with emoji, maybe you could argue they are rather like text instead. But then, once again, they should be consistent with sending text and emoji, both of which use the send button.
This is also important. People who are very dexterous should not forget about those who are not.