In most anime, most of the time, the characters wear the same clothes. Why is that?
I'm sure they are easier to draw but are there any other reasons?
There's many reasons -
Many manga artists draw all their characters with a standard male or standard female face. The only way then to distinguish their characters is by hair style and clothing, if they wear a recognizable outfit.
Branding - as other people said, characters become more recognizable, like a brand when they always wear the same clothes.
Coming up with new outfits is hard. Manga artists have really strict deadlines, so it's important for them to save time when they can, and designing new outfits that aren't terrible take time.
For those manga that have the same clothes all the time, their intended audience isn't really interested in fashion, and fashion isn't really a focus of the manga, and so it's not really worth it to invest time/effort in coming up with new outfits anyways.
I actually think more economical merchandising is not a motivating factor. For every outfit you come up with you can sell another toy to the same group of people who bought the last toy (Look at Card Captor Sakura, with lots of merchandise of her many outfits).
Shoujo manga publishers are much more likely to pressure artists into drawing new clothes regularly because they're more likely to have a storyline relating to fashion (like all those "become an idol" manga), and their intended audience is more likely to be interested in fashion (why else would they be reading a manga where that's a focus?) For example, I know for a fact though the manga author for Skip Beat! was not particularly interested in fashion, she was pressured to come up with new trendy outfits for her characters since she was, after all, writing a manga about movie stars.
I believe that is to keep familiarity with the character throughout the series. Plus when they do add a change no matter how small it's usually very noticeable giving them a significant definition without actually changing the overall appearance.
As @eric mentioned it's not unique to anime. One of the things to consider is most of the anime and cartoon's originate from sketches and that makes it relatively difficult to iterate with significant changes.
Also, it saves time and money with editing and reconstruction while developing it into moving picture.
You could justify it by utility, like warriors not being comfortable with wearing other things in combat/not being able to store their weapons/supplies as efficiently and such. Or by making it a sort of uniform. Literal, or like making a self-conscious character who knows they're forgettable, so they wear the same thing everyday to avoid being not recognized. Or to draw attention. Or something. Or in the case of ghosts, it may be the outfit that they died in. Or shapeshifters - having a limited understanding of society, so they see nothing wrong with wearing the same stuff.