I've been to Canary islands and, of course, Bangkok. You are not comparing like to like. Bangkok should be compared to Madrid and Canary Islands to Thai islands like Ko Chang or Ko Samet (and Ibiza should be compared to Phuket. Ibiza is obviously a lot more expensive).
Now about groceries, they are not a huge part of the budget for many people, so it it's not a huge deal that a piece of salmon or steak costs much more in TH than it does in Spain because unless you're on a tight budget, you compare that price to the cost of going out to a nice restaurant, and it's still cheaper. Same thing with alcohol.
Health insurance can be an issue but not everyone needs medial care often enough to worry about that. Thailand still offers reasonable prices for health care.
Finally, check numbeo.com and compare the cost of living in Bangkok vs Madrid. BKK, if i remember correctly, is about 42% of NYC cost of living, and Madrid is more than 50%.