I agree with you somehow, yet general state elections would be subject to high risk of people not giving 2 shits about the country they live in.
Right now you need to go through a real process (and even have to pay) if you want to do that. Most people won't bother. But imagine living 2 years in Spain and you vote, then another 3 in Germany and you vote, etc. You can literally mess up a country, more than it is/would be.