The counting of the omer is linked so closely to the bringing of the omer. Why is counting a mitzva if we count from the harvest of the barley for the omer offering and we do not harvest that?
Vayikro (23:10-11) "When you come to the Land which I am giving you, and you reap its harvest, you shall bring to the kohen an omer of the beginning of your reaping... the kohen shall wave it on the day after the rest day."
Vayikro 23:15 "And you shall count for yourselves, from the morrow of the rest day from the day you bring the omer as a wave offering seven weeks; they shall be complete."
Devorim 16:9 "You shall count seven weeks for yourself; from [the time] the sickle is first put to the standing crop, you shall begin to count seven weeks."
(Thanks to Chabad.org for the translations.)