Coming to Shul without a hat and jacket is like going to a fancy restaurant wearing a sleeveless T-shirt, shorts and sandals with no socks. (Actually, coming to Shul without a hat and jacket is worse.)
You wouldn’t complain about a dress code to come into a store or restaurant or the White House. Coming into the House of the King of King’s demands a high level of respect.
Just because society has gone to pot, with anyone or everyone thinking dressing down is okay, does not mean a Shul needs to get with the times and welcome you with your sandals, shorts and t-shirt. They appropriately demand you dress befitting meeting the King of King’s.