There are limitations to the packet tracer. This seems to be one of them in which the switches do not support the
aaa new-model commands. Funny thing though is that routers and the Layer3 switch that you mention does support the command.
This will always be a limitation of any simulator. Some simulators will also report "canned" output. This means a display of formatted
show xxxx but it's not actually a report of what is happening on the switch. If you check your help file on your packet tracer, you'll see what commands are supported and which ones are not.
Even though the Cisco IOS is a not a full version of the IOS AND that Cisco doesn't recommend it being your only study tool. I do believe that you can use the Packet Tracer to help you achieve the CCENT and CCNA.
*if the post above has answered the question, please mark the topic as solved.
**All "answers" and responses are offered "as is" and my opinion. There is no implied support or guarantee by the ITProTV team.