The XP admin level account provided full administrator rights all the time. Note that it did not provide full access to everything, such as the "System Volume Information" folder. The always on admin rights was a well known security risk but in 2001 it was considered acceptable.
But the situation changed between 2001 and the time when Vista was in the design phase. Security became an ever increasing problem and Microsoft was under pressure to do something about it. The good news was that computers had more resources. The specified minimum RAM for XP was 64 MB and there were those who thought that too high. But it wasn't enough to provide sophisticated security features. But for Vista it was decided that the minimum RAM specification could be reasonably increased to 512 MB (for Vista Home Basic) and 1 GB for other editions. UAC was one security feature that was now possible. It was a reasonable compromise between convenience and security. Security always has it's price. And not just with computers.
The designers of any complex software, and any modern operating system is very complex, must make many decisions. Often those decisions will be hard. Sometimes they will be very hard. Sometimes no matter what the decision there will be those who say it was a stupid one. You can't please everyone.
Providing more options to the user doesn't solve the problem. This adds complexity and a whole new set of problems. And more decisions for the designers.
The large majority of computer users know little of computers and don't want to learn. The security of UAC must be the default or it will remain off and be useless for the majority of users.
But there are those who will consider any compromise with convenience as unacceptable. For those individuals, and for the rare but real situations where a full time admin account is acceptable, there are options that allow this. I will not describe how to do this.
But if you choose to do this you must accept the consequences and things that will not work. These were the difficult decisions that Windows designers had to make.