Steam is a digital distribution service platform, which in practice means the games are leased to users only on Steam, without granting them any kind of real ownership or ability to run the software outside of Steam.
In particular, below is an excerpt from Steam Subscription Agreement:
A. General Software License
Steam and your Subscription(s) require the automatic download and
installation of Software onto your computer. Valve hereby grants, and
you accept, a limited, terminable, non-exclusive license and right to
use the Software for your personal, non-commercial use (except where
commercial use is expressly allowed herein or in the applicable
Subscription Terms) in accordance with this Agreement, including the
Subscription Terms. The Software is licensed, not sold. Your license
confers no title or ownership in the Software. To make use of the
Software, you must have a Steam Account and you may be required to be
running the Steam client and maintaining a connection to the Internet.
The Software is defined at the beginning of the license:
- REGISTRATION AS A SUBSCRIBER; APPLICATION OF TERMS TO YOU; YOUR ACCOUNT
Steam is an online service offered by Valve.
You become a subscriber of Steam ("Subscriber") by installing the
Steam client software and completing the Steam registration. This
Agreement takes effect as soon as you indicate your acceptance of
As a Subscriber you may obtain access to certain services, software
and content available to Subscribers. The Steam client software and
any other software, content, and updates you download or access via
Steam, including but not limited to Valve or third-party video games
and in-game content, and any virtual items you trade, sell or purchase
in a Steam Subscription Marketplace are referred to in this Agreement
as “Software”; the rights to access and/or use any services, Software
and/or content accessible through Steam are referred to in this
Agreement as "Subscriptions."
In case of retail games requiring Steam to play, Steam is in practice a DRM system, which aims to eliminate piracy and game trading in exchange for a host of online features (multiplayer, achievements, updates, etc.).
Again, to reiterate - there is no game ownership here, you do not physically own the game. What Steam users own is the right to use the software on Steam as defined in license, and there is no automatic/built-in way to play the games outside of Steam (some developers may grant such possibilities, but it is not a concern of Steam/Valve).