The reason is that the term "double" is something of a misnomer. It means "increased penalties" but usually more than just "double." Because your question would be "on point" if the penalties were for a doubled contract were exactly double those for an undoubled contract.Non vulnerable, that would mean penalties of 100, 200, 300, etc. for 1, 2, 3 undertricks, etc.
In actual fact,, only the penalty for the first undertrick is doubled, from 50 to 100. The penalties for the second and third undertricks are 300 and 500 (instead of 200 and 300). After that, each undertrick costs 300 points, so the fourth undertrick is 800 points and the fifth is 1100 points.
This penalty structure was set up to prevent just the kind of behavior you are describing. Under the "old" scoring, doubled undertricks cost "only" an additional 200 pounts (or quadruple 50) after the first, meaning that the sequence was 100, 300, 500, 700, 900, for the first five undertricks. Under these rules, it was worth going down five (win eight tricks) to stop a slam, now it is worth going down only four (800 points versus 1100 for the fifth undertrick and 930 for the slam). Whereas, if "double" meant exactly that, as in your example, it would indeed be worth going down eight (bid seven, make five tricks out of 13) for a penalty of 800 (twice 50*8) to stop a slam.