Windows 7: Why don't we boot clean?

This would sure make things run faster, so why not clear startup items & hide all Microsoft services, ecxept for the one I read we need which is Windows Installer service?

That's much like asking why we don't save gas by not turning our cars on. Disabling services, especially Microsoft services are a good way to render your computer unusable. Some items, you absolutely want to run at start up, like AV software. Others, you may not want to run at start up, such as IM programs, etc.

Follow the steps here to safely establish a Clean Boot that benefits performance: Clean Up Factory Bloatware - Windows 7 Forums