Like all things I don’t understand, I suspect fashion was heavily involved when the decision was made.
Also, ß is ss and since words don't start with Ss there is no need for an uppercase ß. When whole words are capitalized ß turns into SS.
Of course there is a little edge case where capitalization isn't reversible. For example, is the capitalized version of the name MASSMANN to be converted to Maßmann or Massmann? That is in my opinion the only reason the uppercase ß was added to Unicode. To resolve this ambiguity. It has no place in proper German typography.
The Dutch keyboard layout is effectively US international. There are no special characters that that keyboard supports; including all the accents/modifiers we inherited from French, German, etc. Spelling without those is now correct.
This is something that has evolved over the last decades. In the eighties, people would memorize the character codes to produce letters with those. I remember having a card with the right key combinations for word perfect. This is not a thing anymore. People just skip it and the grammar and spelling rules were actually modified to not require these anymore in most situations.
Funnily enough, the Y, is not commonly used in Dutch and usually referred to as the Greek IJ but pronounced the same way.