Why we can cultivate hybrid seed?

Question

Why we can cultivate hybrid seed?

Answer 1

Why "can" or do you mean "why do we"?  We use hybrid varieties because of their ability to produce higher yields or be better adapted to certain conditions.  Hydridization involves inbreeding certain lines of plants.  This concentrates certain genes while getting rid of other unwanted or less desirable genes.  In short, inbreeding reduces the randomness of genes.  Vitality and fertility is further reduced as the line becomes more inbred.  

Then two inbred lines with desirable characteristics are crossed.  For example, one line of corn may have large ears with weak stalks while the other has tiny ears but very strong, healthy stalks.  When the seed that results from that cross (F1) is planted, those plants will be very uniform in appearance and will have large ears on strong stalks.  If you plant the seed from the F1 plants, the resulting plants (F2) will be less uniform with some having weak stalks and others having small ears.