Mt Gox is notoriously hard to get money onto; they require a lot of paperwork and have a huge backlog for processing that paperwork. I guess other exchanges end up with higher prices as a result.
I don't think you are 100% correct. I get my USD into my MtGOX account in less than 30 minutes. I use BitInstant's service, which uses ZipZap, which uses MoneyGram... to get USD into my MtGOX account.
The only thing I hate about the process is that I have to physically go to a store and I have to eat a 4% fee + $3.95 per deposit that is limited to $500 per transaction and $2,000 per day.