Several reasons, I reply. First, if I were in my twenties and had years to send out cover letters and wait for (usually) no replies, finding an agent and hoping the agent could find a publisher and waiting through edits and the year-or-so lead time to see a published book might be worth it. As I am 74, I doubt I have time for all that.
Secondly, if you look over agents' websites, they make it clear that they have many inquiries and must first look for reasons to discard most. My age and the length of my novels are two immediate disqualifies. If any agent reads this and I am wrong, please contact me!