New to PCB design - why doesn't auto placing components exist?
I have been designing some PCBs recently and I would suggest you NOT to use auto-placer or auto-router for your final product. (Proteus has auto placer.)

First of all - Your software is as intelligent as an earthworm when it comes to auto placement or auto routing. In other words, it's dumb as a potato.

Auto routing would not know which placement will get you a better trace pattern which will enable you not only to make an efficient design but also to minimize noise in the circuit. Similarly auto routing doesn't know that shifting a component slightly to the left or right would enable you to route a track in a better way. These tools will just give you a design which is correct according to the circuit. But when it comes to real world performance, things are different.
For example:


Decoupling capacitors should be physically close to the IC.
There should be loops of least enclosed area.
Ground plane should be as solid as possible.
No interfering signals should be close to your crystal oscillator
etc.


Your software won't respect these concepts because these are not mentioned in your schematic. You will know only when you have got the PCB manufactured and it doesn't work as expected all the time. I am not saying it won't work. It might work, for 90% of times but you have to take that 10% into consideration too.

My suggestions is that you should learn some PCB designing concepts and try placing and routing on your own. You can always post your schematic and board layout in forums and experts will give you their opinion/suggestions.