The principal reason is that it would violate the intent of the appraisal statute. Allowing a non-performer to "ride" the efforts of other team members and accrue all the entitlements that "Fully Successful" performance conveys would violate the fundamental principle of individual accountability on which the statute rests.
Secondly, a fundamental principle of compensation policy and practice is that adjustments to basic pay operate at an individual level. Within the Federal compensation system, periodic within-grade pay increases are granted on the basis that the employee, not the employee's team, is performing at an acceptable level of competence as reflected in a rating of record.
Finally, no Federal system can be viewed as credibly managing performance in the eyes of Congress or the American public without making it possible to identify and deal with poor performers.
As noted in answers to other questions in this document, however, summary levels (or other performance distinctions) above "Unacceptable" can be based largely on team performance. Alternatively, a determination that performance is "Fully Successful" can be based solely on an individual's contribution to the team.
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