for one, both are setting records and breaking their own. Both are young athletes and have both yet to unlock their true potential, hence the numerous record breakings.
(as a side note, many of the other records that are being broken are also done with advanced swimsuits. Track spikes on the other hand have pretty much reached its technological peak. Pretty much every sprinter on the world stage are equal in terms of equipment)

second, the rate at which records fall is also inherent to their respective sports. Track records, esp the 200 and 400, are much harder to break and thus their margins are reduced only by small amounts. Thats bc sprinting has a natural human limit. Once you have good form, once u are in top shape, you can only go so fast. Swimming i think is a little different. Due to the technicality (bad spelling i know) of swimming strokes, there is more room for improvement thus allowing for bigger margins of improvement on world records.
On the hand, while Bolt has been blasting world records, he is also doing them at large margins, unprecented by any track athlete in the history of running. Going from 9.69 to 9.58 is crazy and you can bet again he is going to lower the mark in the future. But why would both Bolt and phelps continuously smash records? Because I believe they are the most genetically gifted humans for their respective sport. Phelps has a perfect swimmers body: long torso, long arms, short legs, powerful shoulders. Bolt has a perfect sprinter’s physique: super long legs, super long stride (note since MJ had pretty long legs but a relatively short stride) great upper body strength in balance with the rest.
Any taller and i think bolt would start slowing down

and both have done the unthinkable.

Bolt broke the 200m record. I, along with millions of others, thought it was impossible to break. And why not? the next fastest time every was barely a 19.5 and this was after 14 years the record had been set. MJ ran the second 100m of that race in nearly 9.23 seconds. (which given bolt’s faster top speed and better acceleration, i think his ultimate mark will be around 9.40) So bolt is “infamous” for breaking one of track’s ultimate records

but lets not forget, so has michael. He broke the previously “unreachable” record of 7 golds in a single games set my mark spitz in 1972. Nobody thought that record could be broken but somehow phelps did.

Additionally, swimming has never had any super super star like michael johnson. Ian thorpe had many great marks in most of the freestyle events but in none of them was it talked about that he had swam faster than any other human on earth before him such as was said of michael johnson and his running speed

regardless, lets sit back and watch these two forever ingrain themselves in world history