Cisco did sue Apple over the trademark for iPhone[1]. When Apple later used the name iOS they came to a private agreement with Cisco[2]. If they just used the same name without coming to terms with Cisco, they surely would have been sued again.

[1] https://www.cnet.com/news/apple-cisco-settle-iphone-trademar...

[2] https://appleinsider.com/articles/10/06/08/cisco_licenses_io...