GWAS studies are primarily conducted on germline variants for several reasons:

• The "population" level analysis of GWAS requires that the variant of interest can be inherited.
• Most sampling approaches for collecting the necessary genetic material must be non-invasive to get the large number of participants needed for sufficiently powered GWAS.
• GWAS also requires a control group to calculate risk scores; you can't sample somatic variants in tumors from people that don't have tumors.

TCGA has whole genome sequencing from tumor sites and normal (e.g., blood derived) for many cases. However, these data are considered Patient Health Information under the US Health Insurance Portability and Accountability Act and are not publicly available. Access to these data through TCGA is highly regulated. The application process is summarized here: https://gdc.cancer.gov/access-data/obtaining-access-controlled-data