After applying for patented technology in one country, why do you need to apply in other countries?

It's simple. The government of every country is like a local underworld. Just because you pay the protection fee in this city doesn't mean that you will be protected in another city, unless this new place has an alliance with your original place and will give you some benefits, such as EU patents, but a single country is different. Of course, if one day an underworld unifies all the sites (the earth), this problem may be solved.