Because the two consonants at the end of paard and the single consonant at the end of schaap.
If you were to say parden you get a "short a" sound, which is caused by the two consonants following the a. To keep the "long a" sound of paard you need to keep the two aa.
Schapen has only one consonant following the a which creates a "long a" sound.
Have a look at this website on different sounds in dutch: http://www.heardutchhere.net/DutchPronunciation.html