Often cancers have few symptoms until late. My daughter died last year from kidney cancer. She had no idea until it had metastized to her spine and hips, causing excruciating back pain. Because she was young (45 when diagnosed) and female, her doctor basically ignored her complaints and told her to take Tylenol and sent her to PT. It wasn't until she discovered a lump in her breast that they actually did tests and found that it was spread all over her body.
I'm sure that like most older men, Pres. Biden had urinary symptoms of an enlarged prostate. I'm also sure that he had his PSA checked at least annually. It sounds as though this "aggressive" form of prostate cancer wasn't detected until it had quickly spread.
May the gods bless him and his family. Poor, poor guy.