I understand it might look as though they haven't given their presents much thought, but it's hard to know what's in other people's minds - even our nearest and dearest. If you've generally got a good relationship with your kids you maybe just need to put the gift buying to one side, have a wry chuckle about it, make a point of buying yourself something you really want, and/or find a tactful way to change things. We agreed a few years ago to focus on the grandkids at Christmas and, although we usually give the adult children with kids restaurant vouchers, ask them not to buy us anything. I ask the adult children and the grandchildren to put a list of things they'd like on Amazon for birthdays (and Christmas for GC). Items can be bought elsewhere and you can ( invisibly to them) remove the item from their Amazon gift list if you prefer to shop locally or find it cheaper elsewhere.
Maybe you could suggest everyone uses gift lists or you could indicate in advance of your birthday if you have a charity you'd like them to donate to instead of giving you a present. Whatever approach you choose, try not to let this one issue cause resentment or a fall out - there's too much to lose.
