And what has german got to do with it. (or pronunciation for that matter, in dutch ei and ij are pronounced basically the same, but just like your comment that hasnt got anything to with spelling. Lijden is something different from leiden even though they sound the same)
Our ij sounds exactly the same as your y in by (if you want to bring up pronunciation as an argument)
If it did start with i as a single letter and not the combined ij we would pronounced it very differently more like in knee (basicly like we pronounce i+e in dutch, like in "niet")
"I and J at the start of the name are both capitalized because the combination is a digraph in Dutch and therefore treated as a single letter."
It is just like æ or how the w formed out of vv (so not like sh or th that do make different sounds in those combinations, so an actual letter, not just a sound.)