Are you running MAMP on mac or windows? Not sure it matters, as they should be configured similarly. I've used MAMP on both and never had any trouble that you are describing. I can guess what the problem is though.
My hunch is that you are either using an Apache virtualhost setting or have made a modification at some time that has:
Either for the default host, or the virtual host you are using MAMP Pro, you can check these via the admin. These are the default settings, and you can see AllowOverride is set to All.
http://polydeon.com/monosnap/_MAMP_PRO_2015-02-22_18-29-14.png
If you are using regular MAMP you might have to edit the httpd.conf file manually.