What does patent limit you to? How can there be many manufacturers of a certain product?
Patents are granted for an invention as defined in the “claims” of a patent. If a competing device falls within the scope of the claims, then they infringe the patent. If not, then they are not an infringement. If a competing device does not infringe the patent, then that patent cannot be used to stop the competing device from being sold.
Claims are set out in a patent and infringement either occurs or does not occur depending on what the claims say. There is often a difference between what the general public consider an invention to be versus what the patent actually covers.
Some patents are more easily avoided than others. A large amount of time and effort is spent by competitors to design around patent claims so their devices do not infringe. How a claim is drafted is vital to the scope of a patent. If properly drafted by a patent attorney then the claims will give a fairly broad scope of protection. If not properly drafted then the claims are easily avoided. If the prior art (the things published and used before the patent) is very close to the invention, then this also causes the claim scope to become very narrow so that it is easily avoided.
A patentee can also license the patent to a variety of manufacturers. Other possibilities for why there are competing products are: the patent is not valid, not enforceable, has expired or lapsed, or may be under litigation.