The European laws have specific sections regarding digital goods. The following two passages are relevant to you:
From Returning unwanted goods:
Please note that you may not use goods that you have received before deciding to withdraw from the purchase. The right to withdraw exists to allow you to examine the product in the same way as you would in a shop, not to give you 14 days free use.
Be aware also that more specific rules apply to digital content (e.g. downloading or streaming music or video).
From Shopping online:
Specific information requirements apply when you buy digital content online, e.g. when downloading or streaming music or video. Before you make the purchase, you must also be informed how the content operates with relevant hardware/software (interoperability) and about its functionality, including whether any geographical restrictions apply to the use of the content and if private copies are allowed.
You also enjoy the right of withdrawal within 14 days from concluding the contract for online digital content. However, once you start downloading or streaming the content you may no longer withdraw from the purchase, provided that the trader has complied with his obligations. Specifically, the trader must first obtain your explicit agreement to the immediate download or streaming, and you must explicitly acknowledge that you lose your right to withdraw once the performance has started.
So yes, the law specifically allows you to waive that right when purchasing digital goods. So long as Steam has correctly advertised the product's system requirements and other key details, you lose your right to withdraw from the purchase the moment you start downloading it to your system.