Why isn't a license shown when installing (and when installing most software)?

As a former Windows user I was used to accepting EULA's and other licenses when installing any piece of software, or when installing the whole OS. On Ubuntu it happens extremely rarely (I've met it only once). Is it correct? Ubuntu and most of it's software is under some free-to-use-and-modify license like GNU GPL or something similar, but shouldn't the license be presented to the user?

This is regarding free software:

A license is very different from a EULA. The software on your computer starts out under the normal (default, in some countries) copyright by the authors. Only then do you get special permission to copy and re-distribute the software under the terms of the GPL. This does not restrict your use of the software, nor does it put any special onus on you to comply with some arbitrary terms. It is not a contract.

Think of the GPL as an agreement of the terms of re-distribution; you are granted license to deal with the software in ways normally prohibited by copyright law, given you agree to some terms.

A EULA restricts your use of the software, it has nothing to do with simple copyright. The EULA says, you may use this software, but not in any way you like. It restricts you further than copyright or trademark law could. Free software doesn't have that kind of restrictions, therefore you don't have to agree to any contract.

I wasn't able to comment on Stefano Palazzo's answer above, but I just wanted to clarify that it's not necessarily true to say that an open source licence isn't a contractual licence. They often are, and it may well be the case that all open source licences are contracts in some jurisdictions.

But of course he's right in that there are generally no obligations placed on the user so there is no incentive to get them to accept the contractual licence. Copyright law provides an adequate fallback for any shenanigans, so there really is no need for an open source software developer to inconvenience the user with an unnecessary legal process.

By the way, while you're free to not agree with the licence, using the software without a licence will leave yourself open to a copyright claim from the developer. Generally they wouldn't act on it anyway.