Referring to the question here, the first and second paragraph.
why FIFA not just decided to start all four matches simultaneously?
Because that reduces the television audience. Having two matches at the same time is bad enough, four matches just makes it worse.
Also note that in this case, it’s not the round of 16 game (which is group G vs group H) which is the issue, but the quarter-final. You’d have had to have played eight matches (everything from groups D, E, G and H) to avoid the issue.
FIFA can consider to let all the qualifying teams to draw the lots again after all group matches complete, so that no one can “choose the opponent”, why this is not done?
Because it’s awful for fans. As (say) an England fan, at the moment I know that England will play their round of 16 game in one of two locations, and the companies arranging travel / hotels are able to handle two possibilities for the match. With a random draw, England could end up in any of eight locations, which would make it much harder for fans and travel companies to arrange.