It is "easy enough" to explain the basic question with a ray diagram or similar means - see below
BUT it is important to realize that the answer to why viewfinder or human eye images are not inverted is "by design" or "because" (choose one, both the same essentially). That is to say, the system requires the outcome to be a certain way, so whatever steps are required to implement the outcome are provided.

In the case of a viewfinder, extra lenses, mirrors or prisms (or a combination of these) are added as required to achieve the final result. The real question becomes not "why is this this way up" but how is this done.

In the case of the human eye, the image on the Retina IS inverted and the brain looks at it the "right way up" as far as the viewer is concerned.

The information below from this excellent site shows how basic inversion works.

Also see -> More on Ray Diagrams

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In the case of the eye, the image is inverted:{ From here - low tech but interesting }

http://www.quantumtheatre.co.uk/Lights%20&%20Sounds%20notes%20Key%20Stage%202_files/image022.jpg

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IMPORTANT:

Note that while the above image grabs your attention as it demonstrates inversion, it actually does a very bad job of showing how the eye lens works. As the eye lens is increasingly embedded in the cornea, the air-cornea interaction does most of the 'lensing' while the cornea-lens interface only manages about 10% of the total bending.

An excellent discussion of this is available here - see the In your eye and a reasonably correct image of how light is actually bent by the eye is shown below.