For the same reason I didn't convert the equations into photos and store it in .doc format... because it is more editable. PDF is great for completed works, but not so good for a work in progress.
That's fine but post it in PDF as well for those who don't have the latest version of MS Office.
As for open source alternatives; THEY SUCK! Open Office is crap compared to Microsoft Office.
That's a matter of opinion.
I personally don't like lots of the MS Offic programs: MS Word and Publisher are crap, Excel mediocre and PowerPoint poor. I find the interface is horrible and performance poor. The computer I use at work has MS Office installed and I very rarely used it and run Openoffice most of the time. I wouldn't use MS Office, even if it were free.
Go cry to your mommy if you don't like using Microsoft products just because they are a business and not hippies who believe it's wrong to make money off your good work.
I don't mind some Microsoft products: I'm currently posting from a PC running Windows, am perfectly happy with it and wouldn't choose to run Linux (tried that and wasn't happy with the performance) and have never tried Mac OS because I've no reason to.
I don't mind Microsoft being a business: I use other commercial software such as LTSpice and AutoCAD and some of their software is quite good: Visual Basic was great (not tried their Visual Studio yet) and I liked MS Access the last time I tried it; Visio is reasonable, although I don't know how it compares with Corel Draw or Adobie illustrator these days.
You should get your facts straight:
OpenOffice.org was originality developed by a business (StarDivision) who were bought by Sun Microsystems in 1999, then by Oracle in 2010, who in 2011 discontinued support after alienating the developers and is now developed by the Open Document and Apache Foundation: non-profit organisations. I had no problem using OpenOffice.org when it was developed by a business and it's just as good now.