On the first birthday that I shared with my husband, because he had to go to work before I woke up, he left me a pile of gifts on a chair in the living room, only thing was, he hadn't wrapped any of them. I duly thanked him for the gifts, but it really bothered me that he hadn't taken the trouble to wrap them, so in the end, late that night when he was asleep and it was still niggling away, I went downstairs and wrote him a note telling him how disappointed I was. He said that he hadn't realised it was important to me and has never done it again. So maybe you haven't made it clear to your kids that you like to receive cards from close family, and while you shouldn't have to, perhaps a remark near your birthday next year, saying something along the lines of 'I hope you won't forget my birthday card this year', will be the gentle reminder that they need. If that doesn't work, I'd cease bothering with their birthdays, and definitely don't be so available to help them out, as your efforts obviously aren't as appreciated as they should be.
You are definitely NOT being unreasonable, and I'm sick of all these excuses about the younger generation being so busy, they know the date of your birthday, and have all year to prepare for it and buy, and post the necessary card that means so much.