Imperfect does not mean ineffective. Every time you make something more difficult it reduces the number of people who will do it.
Pardon my 1990s metaphor, but:
* If you have no DRM and people can just share the install disk, they will do that and piracy will be universal
* If you implement a CD check, yes, people with CD burners can bypass it but those are far fewer. Yes, industrial shops can mass-produce pirated CDs but not everyone is willing to buy those.
* If you implement even more stringent restrictions such that duplicating the CDs is significantly harder (to continue the metaphor, do something weird with the sectors that requires CloneCD instead of more generic ISO-ripping software) and now you're down to people with specialized hardware/software
* If you go further and implement software DRM checks, they can be bypassed, but now we're down to the portion of the market willing to download sketchy crack programs that totally aren't viruses, the host of the website swears. This is a *much smaller* group than those that would just grab an official install disc from their friends.
etc., etc. These measures do not have to be perfect to be effective. There can still be pirated copies available, but if the effort to get to them is sufficiently higher than buying the official copy (and that threshold is different for different people) they have served their purpose.
Most techie people I know ripped their DVD collections. Many ripped their Blurays but plenty didn't because it requires specialized software to get around the DRM. Only a handful of them have ripped their UHD discs which require specialized software AND specific hardware AND flashing a specific firmware on that hardware.