Why are too many plugins bad for a site’s speed?

I always thought it’s because different plugins load different scripts on a page. So I optimized some of my plugins (that otherwise load scripts and css files on the home page) to load them only on specified pages by hacking the plugin files with conditional tags. So now on my home page I only have my style.css and the feed’s xml file being called. But is there any reason why just having a lot of plugins activated, will slow down a site (or at least the homepage even if extra files aren’t being called on the homepage).