Google indexes and ranks individual pages so it's possible to SEO individual pages within the site.
But a major part of rankings success lies in the votes (links) that other 3rd party sites give you and by far the majority of those IBL's will be to your home page URL. Sure you can ask for the link to be diverted to your internal money pages but that is a robbing Peter to pay Paul scenario.
An internal page is typically about one subject and is part of a broader site. eg... the Xanadu car hire page could be just one of the 300 pages that make up the Xanadu vacation guide.
Despite some very focused SEO on my part for that (fictional) Xanadu car hire page, it is always outranked by the home pages of sites where the whole site is specifically about Xanadu car hire.
Do internal pages face an uphill battle to get top rankings in competitive niches?