What has led to this experience? On the top of my head I can see the following reasons:
* Release Often as KPIs for developers
The release often KPI for promotion and bonuses has led to constant changes to 'systems that are working fine' to become ever-changing user experiences. While daily users can gradually phase-in changes, most sites that are casually used will confuse users with completely new error-prone experience.
* Payment Security and Financial Regulations
At least in the EU fraud has led to various tech-related regulation calling as an example for separate apps for IDs and for transaction verification. While it is well-meant, it leads people to check bank statements less often and anecdotally in my family confuses especially elderly users to the point of introducing more opportunity for scams and fraud.
* Patch-work nature of ID & Verification
Captchas, Two-factor SMS, password rules and Authentication Apps have been patched onto the original user/pass system. The experience has become truly annoying with some clear winners: anecdotally more and more people simply use Google/Facebook OAuth as logins to sites. This is fine from a UI perspective, but lacks consumer regulation - what happens if you lose your access and who can you contact if your accounts get compromised/scammed/blocked?
* KPI switch from customer first to business model first
Having gained their audience share, Amazon and Google have switched from a 'customer is king' perspective to one which suits their business model most.
What are other reasons?