Because he's going to get way more than 10/300, and it could very well be front loaded anyways.
Harper is going to get both the chance for a huge short deal and the security of a $350M total deal, so he doesn't have to settle for $200M guaranteed.
Some team is going to buy the rest of the median projection WAR Harper is going to produce for the rest of his career (40+ WAR if he is a true talent 5 win player now and follows typical aging curves). It doesn't matter if they spread that commitment over 10, 12, 15 or 20 years. In fact, most teams bidding for Harper will prefer the longer term to lessen the impact on the luxury tax threshold.
Harper is going to get the chance to decide to cash in again if he actually becomes the monster everyone thinks he can be, despite only really being a superstar for 1 season. If he turns into a true talent 8 win player over the next couple years (basically ARod, but less than Trout) he will be able to sign another contract buying the rest of his projected WAR...but this time as an 8 win player rather than a 5 win player.
40+ WAR is $400M+, minus some amount of value assigned to the opt out clause(s). He will blow by $300M guaranteed, and won't even consider something like 5/200...unless he just really wants to be a Dodger.