I love ET conspiracy theories. Please enlighten me what is not kosher or what the angle is.
There is a whole demographic that increasingly only use smartphone apps and don't want or need a desktop app equivalent. Perhaps you aren't the target demographic for these brokers.
I suppose you would suggest the same conspiracy about the new challenger banks that are also mobile-first? eg. Mondo:
https://getmondo.co.uk/
I think a more likely explanation is that this is what the future will look like across the board not only for banking and trading but for all kinds of verticals.