I have to admit I’ve had all the same thoughts as the OP given that the Anglican Church did not seem to be founded on a strong doctrinal difference. I can much better understand the people who went off to be Lutherans, Methodists and Presbyterians.
As far as I can tell, many people in UK were Anglican because it was what they were born into and there were also feelings of nationalism involved. Anglican was 'their church". And obviously in some parts of the UK in past eras, being an Anglican got you access to jobs and social position. Catholics were looked down on and shut out.
With respect to the US Episcopal church, for many decades it was essentially the church of the prosperous people. If you wanted to go up the social ladder, the Episcopal church is where you wanted to end up, to worship with other rich people, similar to Trump’s “prosperity gospel” church.
In more recent decades, the Episcopal Church in US has been framing itself as the super-liberal church where they ordain women, ordain gay people, march in support of abortion clinics, etc. so it is also getting members from the groups associated with those kinds of activities. I have also seen some of this with the Anglican Church in UK but have been told by some people affiliated with Anglican Church in UK that not every Anglican is on board with the liberalism or supports it and that the Anglican Church in UK is really different from Episcopal Church in USA…oooooook, whatever.