Because it's not trivial. :)

Most loss of function events will be non-synonymous mutations in exonic regions which either significantly truncate or frameshift the resulting protein. You don't really know whether it's loss of function without doing validations or looking at protein models or crystallography to assess structural features. SIFT / ClinVar scores etc are usually good indicators of these types of events. Same really goes for gain of function. What you're likely looking for are mutations with high significance annotations (i.e., likely to result in significant alterations to protein composition, e.g., stop gain event, early frameshifts, etc) and high ClinVar scores for any clinical diagnostics. If you're looking for novel SNP / indels associated with a phenotype, you could do GWAS then validate hits invitro / invivo. But that's pretty nuanced in it's own right. Not something you just pick up in a weekend.