Actually there are 3500 genes that are directly linked to recessive disease and no, not even close to all of those have an existing test. There are also an additional 3500 or so diseases that are suspected to be genetic illnesses but for which the exact gene is not yet know.. Further, the problem with most existing tests is that they are single gene, and each single gene test costs usually at least $1000 - $3000 to administer. The way it mostly works now is that you have a physician or medical geneticist looking at the patient and guessing which single gene test(s) to order. If it comes back negative, the physician makes another try using another gene test. This in turn leads to what is commonly called the 'diagnostic odyssey' where a patient can go years without a diagnosis and can easily go through 20-30k of test.
Contrast that with whole genome testing where for a relatively low cost you can look at all the genes in the genome. This has huge benefits, not the least of which is that genes which wouldn't have normally come to mind for a physician (in the case of very rare diseases for instance) will be interrogated and a mutation can show up.
Disclaimer: I'm in the field working on whole genome sequencing and reduced gene panel sequencing as a diagnostic tool.