Upvote:4
Apparently because the Code of Federal Regulations strongly recommends it:
8 CFR 214.2 - Special requirements for admission, extension, and maintenance of status.
(b)Visitors -
(1) General. Any B-1 visitor for business or B-2 visitor for pleasure may be admitted for not more than one year and may be granted extensions of temporary stay in increments of not more than six months each, except that alien members of a religious denomination coming temporarily and solely to do missionary work in behalf of a religious denomination may be granted extensions of not more than one year each, provided that such work does not involve the selling of articles or the solicitation or acceptance of donations. Those B-1 and B-2 visitors admitted pursuant to the waiver provided at § 212.1(e) of this chapter may be admitted to and stay on Guam for period not to exceed fifteen days and are not eligible for extensions of stay.
(2) Minimum six month admissions. Any B-2 visitor who is found otherwise admissible and is issued a Form I-94 (see § 1.4), will be admitted for a minimum period of six months, regardless of whether less time is requested, provided, that any required passport is valid as specified in section 212(a)(26) of the Act. Exceptions to the minimum six month admission may be made only in individual cases upon the specific approval of the district director for good cause.
Hence I believe CBP Officers (not wanting to bother with having to fill paperwork to defend their decision to give you less than the six months to the District Director) default to it.