It isn't hard to make them triggered a different way, it is that it is hard to make them reliable enough and triggered a different way. They are designed to be triggered this way because it makes them simple. They are hard to service (requires surgery) and need to be triggered reliably when they are bring triggered. To achieve this, they are made as simply as possible, so have a simple trigger mechanism. Users of them are then told that they have to avoid this situation, by being very careful. One of the instructions anyway is to not keep a phone near them, because of concerns about RF interference. It isn't a case of making another way to trigger pace-makers, it is a case of making a way that is extremely unlikely to fail to work and will remain like that for decades, and where equipment to do it is very cheap. It will then take decades for there not to be people with these devices walking around. If they are changed to work a different way, you also need to equip hospitals around the world with a way to trigger them. Basically, it is really impractical, and it is not as much of a problem as you might think because users should already be hyper aware of the danger of this. Anything you do to change it has a huge risk of killing them because of failing to work if it turns out to be even a tiny bit less reliable after the devices have been inside people for years.
Another example of a device that would be a problem like this is my watch. Jewelry that would have similar problems is also common. Things which would be a problem if put close enough are everywhere, but this is always going to be an issue with something like this that is designed to be triggered easily and extremely reliably, and the decision has previously been taken to educate the tiny number of users of them about it rather than go through the unimaginable logistical effort of changing them now (obviously it was much easier when they were new and rare).
Edit: removed paragraph about making a warning device that I don't think is worth including here.