The following Wikipedia article states that the U.S. is not a signatory member of The United Nations Convention on the Law of the Sea. Why is that? What would the U.S. get to lose by ratifying the convention into law?
The US actually is a signatory since 1994 (following an Agreement on Implementation), it just hasn't ratified the treaty.
To stick just to the relatively recent history, the ratification is blocked by a group of Republican senators, and this has been the case for a good number of years. During the Obama administration, the executive was in favor. The G.W. Bush administration also supported it. By the accounts I found insofar, the Trump administration has been silent about it.
The objections of this Republican group can be pretty hard to discern in detail. According to a 2018 Icelandic PhD thesis on the topic:
Intriguingly though, the U.S.’ specific resistances to UNCLOS have muddied – as opposed to clarified – over time. [...]
Now, just what this highly-conservative contingent has against UNCLOS is difficult to discern, a status which is likely unsurprising at this point given the U.S.’ seemingly incongruent stances on UNCLOS. The common talking points which are used in arguments against UNCLOS are largely ideological, ranging from its status as customary international law [...] and fears of the International Seabed Authority (“ISA”) as some “unaccountable supranational bureaucracy that will defy U.S. wishes and redistribute undersea wealth to developing countries.” When pressed for more specific concerns, or even for potential compromises, this Republican contingent has frequently done little more than repeat itself or simply remain quiet.
Wikipedia has longer article on the topic of the US position on UNCLOS, but it kinda stops in 2012.