Do pilot uniforms always include a shirt with stripes? On airliners, pilot's uniform usually include a shirt with stripes on shoulder. I cannot find examples of pilot without this kind of uniform while in service (either in the airport before departure/after landing or on the flight deck). It seems that during long break they can wear T shirts but use their formal uniform when working.

I fail to see why they should wear such a uniform (wearing a shirt with stripes don't make you a better pilot).

For flight attendants, the uniform reflects the image the company wants to convey. Flight attendant uniforms are thus diverse and different for different airlines. But a pilot's uniform is more uniform from one airline to another.

Do airline uniforms for pilots, which do not use shirts with stripes on the shoulder, exist?
Uniforms are to make it easy to identify members of a group who have a separate function or authority from a larger more random group.  The stripes are to easy to identify the chain of command within this group at a glance.

Most of the time it's all just costume peagentry.  

When the shmoozle hits the fan however, being able to identify who's in charge and next in charge and so on, without having to, you know, ask around, becomes a big deal.  That's why the bars.

You won't find a first or second tier carrier in a Western airline without a uniform shirt and rank bars.  I can't find any specific regulations that require that, but it's basically an old convention carried over from the marine world (it's why pilot uniform suits look like naval uniforms), where many of aviation's traditions originate.