The reason is that they are too tall to fit thru doorways, so the options are:
1. Shrink them down which probably takes away the fun.
2. Allow them to clip through the top of doorways which looks ugly and would probably get mocked by players or rival gaming companies and it might also result in hacking issues.
3. Make them only playable via toys or transforms and simply have them get blocked from entering some doorways.
4. Never have them as a playable race at all.
Ogres suffer the exact same problem. Can't fit thru doorways.