Because it is formed by meiosis in microspore mother cell. It is haploid and that allows it to fertilise with another haploid megaspore and give rise to a diploid zygote which will germinate into a mature plant body . (In some cases if the plant follows haplontic cycle the temporary gametophyte can again perform meiosis and continue being haploid like in bryophytes. ) . In angiosperms the haploid spores give rise to diploid zygote ( thus maintains a constant ploidy ) and matures into a plant body.