Shouldn't it be common sense to get a new ship after 15 years, so people can avoid screwing up the oceans more?
Oh, that's what big boys may do -they sell the ship to someone else. And that someone else will use it to make dirty bucks. You know, the ship is old, but still can pass the safety checks. Of course in being old its performace is poorer and it demands more maintenance, so crew costs are cut and maintenance is kept to a minimum (that's standard policy for tankers anyway -don't fix it unless it's absolutely necessary is the rule). Of course, the mix of poor crews (usually poor devils from SE Asia who get paid about 500$ per month) and old ships may lead to trouble, but that doesn't happen often enough for someone taking action. Ships, shall we remind, are still safe enough to be hired & the cargo be insured (that's the crux) and be allowed to enter port...
Yet as they're old they get piss-poor works, are hired for a cheap price to transport cheap cargos (heavy crude, tars, fuel) which aren't worth hiring a newer tanker. And each now and then, the mix of sea (which is a bitch), poor crews and an old ship may cause an incident or even an accident. But, what else would you do? You can't forbid a owner, who usually is a company who owns a company who hires a company who owns a ship -all them based in havens-, to do something legal... and havens are havens because the definition of what is legal includes allowing to sail older tankers as long as they're not caught up in a random check, the insurers refuse to insure the cargo or potential hirers reject them. And even if a tanker is caught up in a random check in a strict country, they may be repaired enough to fulfill the regulations -and not a dollar more- and sail away. Of course whenever they reach the next pory, the travel stress will have taken a toll and the tanker likely no longer will be able to pass a random check... but, it would be so bad if a tanker ran across two random checks in a row! And so they keep sailing, getting their 15,000 to 20,000 $ a day (crew expense, about 9,000 $ a month; fuel expense about 6,000 $ per day at sea, and 1,000 $ when in port; usual taxation 1% of brute income), carrying loads worth 5 to 20 million $ and rendering untold profits once maintenance is discounted.
Old tankers are a way of living to a gazillion of investors, and their legal status usually is so messed up that it takes a very determined Government (and a bit of luck) to ever get a compensation when shit happens.
For an isntance, a wreck that happened in Spain: the ship had been hired by a company based in Cyprus, who was hired by a company based in Panama which was owed by a company based in Panama, which was owned by a holding based in Bahama whose investors had limited repsonsability -and where undisclosed. The load was owed by a Italian company which had boutght it to a Bielorrussian owner which had hired the ship and then sold the contract (for ship and cargo). Half way the load was sold to a third undisclosed customer... and all that is typical for about half of all tankers in the world. Not even the USA inspects all tankers reaching its shores; usually they random check 1 in 20 tankers, which is a lot (even the very serious Dutch random check only 1 in 20 tankers as average).
Oh... and double hulls don't really help much. Double hulls' frames are punished even further than usual tankers and get old sooner, and their external hull plating is thiner to save dead weight. They are safer against spills and minor scratches, but structurally suffer more, their hull is weaker and their maintenance is more demanding. But, nobody is going to pay what it costs a new tanker to carry a cheap load of heavy crude or tars, so the market for cheap and old tankers still exists. And is being filled with egg-skinned double hulls... and in case of structural failure having a double hull means nothing.
