Most WP Theme is "code-nasty"

Hello,
i just starting to provide wordpress based solution to my client a month ago, try to test commercial theme from themeforest as well as the free one available from www.remarpro.com. Previously i’m using the other famous CMS (yeah you can guess it..).. and have been 12 years in web development industry… here’s something that bothering me..

Why a lot of WP Theme does not have a good coding practice.. things like
1. loading CSS Stylesheet on above , loading JS on below
2. loading unused css/js

i believe the no. 2 issue could cause our wordpress site become slower than its potential, right ? i do have several commercial theme.. and i see the css/script loaded in about us page (or almost anywhere).

Can you give me a clue:
Is this because the behavior of wordpress to load all registered plugin upfront ? or just the ‘lazyness’ of the theme developer to make code tidy?

I’m thinking for my site to load the js/css needed on a plugin when it being used..

Thanks
Victor
Wordpress Newbie