I think some of it has to do with portions of the code not being owned by a particular company or the original source of a code being uncertain.
Let's use Word 5.1 as an example. I know not all of the code in Word 5.1 was developed by Microsoft. They would need permission from the other companies involved to release it free of charge even if Microsoft was OK with releasing their portion of 5.1 for free. This may be part of the holdup with some Mac products as well. MacDraw II and MacWrite II both had third party utilities included in the package and I'm pretty sure most later versions of the Mac OS did too. Also, the only fonts available on Apple's page are their four fonts, not the additional ones that came with the LaserWriters.
Lost ownership is another issue. I remember hearing once that this is the reason we may never see the original Oregon Trail and Number Munchers released for free. With SimCity, for example, we know who owns the code--Maxis was purchased by EA and many of the developers remain in contact with today's projects. This is how EA was able to release SimCity for the OLPC project. Oregon Trail and Number Munchers were both MECC products. There were countless buyouts here and lots of things were lost in the shuffle (I actually wrote to one of the Oregon Trail II team members, who was on board for many other MECC products, about some material from the game and he said lots of the original material that was researched for the game, including primary source documents from private families, was lost during the buyouts). If they can lose personal trail diaries from the 1840s and 1850s, losing copyright documents would be no problem whatsoever.
Of course, it's probably possible to look this stuff up in some sort of government-run database, but companies seem reluctant to do that for whatever reason. If the buyouts are documented, the rights holders should be easy enough to track down, although "lost ownership" will still be brought up all the time.
Personally, I think the copyright terms on software should be shortened. 75+ years is fine for books, movies, and music, since they don't become obsolete in two years (unless we're talking about atlases, travel books, computer books, and so forth). For computers, I'd say 15 years would be a good amount of time--most software is out of use by then and having older software in the public domain would help many (historians, people who cannot afford new software, schools, churches, nursing homes). If a company wanted to keep something copyrighted longer (say Microsoft with XP if it was still supported past that date), they could apply for a 5 year extension. There would be no limit for these extensions--if someone wanted a 75 year copyright, they would just have to apply for the extension. Additionally, if this law were to be enacted today, a provision would have to be that a work created prior to this date in 1995 would enter the public domain unless a rights holder stated otherwise, in which case it would be granted the 5 year extension.
This is just a dream for now, but I'm thinking of talking about it with my local congressman to see if it goes anywhere. For now, I'll keep purchasing new copies of Word 5.1 whenever I need to install it on an additional Mac.
