Because there's considerable security concerns. If you need to access via a secure OWA website or a VPN, they can configure your connection to have limited rights as well as at least require up-to-date virus software is installed on your computer. They can reasonably safeguard your connection to the mailserver just like any other remote access oriented service they may provide.
Just giving you free reign to connect to their exchange server would involve opening two considerable vulnerabilities to the enterprise environment:
1. The exchange server and firewalls would need to be configured to just let any old connection through in an attempt to access mail accounts, which means you can connect to it from home, but so can any person attempting to gain unauthorized access to the company network.
2. Outlook is a MAJOR target for viruses, especially worms. A virus ends up on the company network and they can quickly contain it. A virus hijacks your home PCs Outlook and starts spamming all of your business contacts with virus-laden dick pill adverts, sent from your official @CompanyName.com email address. Best case is you hammer the corporate spam filter into oblivion and company mail service slows to a crawl, worst case *they get sent through* and your company just infected 2000+ clients with a computer virus, potentially compromising their personal information and definitely compromising their business relationship with your company.
IT doesn't come up with these rules just to punish employees working remotely, there's serious and legitimate reasons why giving people free access is not typically done.