Contrary what most people may think, isolation is not only due to the material which is used and the perfect seal that one must reach.
Thickness of the material is also key.
With customs, you actually get at least 2cm thick of acrylic/silicone material sealing your ear with a shape following your ear canal.
Because let's be honest, acrylic materials are not exactly top notch in term of soundproofing. Silicones are already better at that for example, which partially explain why you can get excellent isolation with a thin silicon universal tip and a good seal.
Customs make it possible to have good isolation with any kind of material provided they're long enough, and acrylic resins are used a lot because of their excellent mechanical resistance. Silicone customs are also available but they are softer, more permeable to solvants and grease and prone to breakage when they age, although silicone materials have come a long way during the last 10 years. They now age much better (no yellowing with uv, less hardening/drying).