Many companies that are now registrars most probably registered their domains before they were themselves registrars (chicken and egg). In the case of Amazon and Google, these domains were registered many many years before they were registrars (which is only a relatively recent side to their business).
namecheap.com was registered in 2000, but Namecheap only became ICANN accredited in 2007, prior to which they seem to have been a reseller for ENOM (as @davidgo pointed out). And according to
whois.domaintools.com the domain has never been transferred to a different registrar.
Transferring domains is not without risks (
.com domains need to be unlocked prior to transfer) and these high profile domains are probably more vulnerable than most. Although, according to
whois.domaintools.com these Amazon and Google domains have been transferred 2 - 3 times in their 20+ year history.
The current registrar for all 3 of these high profile domains is "MarkMonitor Inc", a company that would seem to specialise in brand security of over 20 years, so they are perhaps more suited to the job:
MarkMonitor Inc. is an American software company founded in 1999. It develops software intended to protect corporate brands from Internet counterfeiting, fraud, piracy, and cybersquatting. It also develops and publishes reports on the prevalence of brand abuse on the Internet.
Of course, the registrar could manage/transfer their own domain(s) if they wish (providing they are accredited for the particular TLD*1), but there isn't necessarily any benefit in doing so.
(*1 Even though Google support 329 TLDs, they don't actually support all the TLDs of domains that they themselves own.)