Some haploid forms are full grown adults, such as male ants and bees. The haploid form of fungi is usually dominant over the diploid form. Haploid forms are not some crippled half version. They are a complete life stage of the organism.

The fact that our diploid forms are what we call people, while the haploid forms are microscopic probably has more to do with how we have sex than anything.

You might argue that the diploid form is more robust genetically, but the sperm cell has great need for robustness.