Why should I buy a synthetic whetstone instead of a natural one? Aren’t natural whetstones better?

Synthetic whetstones are much more quality consistent and cost effective than natural stones. For most of us, buying a whetstone that gives the most consistent results is desired. Because the manufacturing process and exact amount of abrasives and bonding agents are controlled, we can expect the same stones from different batches to perform the same.  On the other hand, natural whetstones can vary greatly, even when mined from the same zone of a particular quarry. Also, because the additional human labor involved and their limited supply, Japanese natural stones are often cost prohibitive for most sharpening enthusiasts.

While there are always some personal preferences when it comes to choosing the make/series/grit of whetstones, natural stones are particularly personal. As always you should learn sharpening with quality synthetic whetstones, do most of your routine sharpening  and polishing on synthetic whetstones, and only explore natural whetstones as "extra curricular" adventures for certain finishing details (if you have the budget).