The argument is that whoever provided you the installer is ‘morally speaking’ providing you the software on the condition that you accept the agreement, and the fact that you physically receive the software before completing the transaction is just an implementation detail.

This is largely based on ProCD v. Zeidenberg:

https://en.m.wikipedia.org/wiki/ProCD,_Inc._v._Zeidenberg

That case involved a literal shrinkwrapped product – a piece of software sold on CD-ROM in the 90s – where the contract was stored on the CD-ROM and thus could not even be reviewed without breaking the shrinkwrap, which in turn could only happen after buying the package from a retailer. If the purchaser read the license and decided not to accept, they would have spent their money on nothing.

But the contract instructed purchasers to return the package if they didn’t agree to it, and purchasers apparently had the opportunity to do so and get their money back, and the court decided that was good enough. If a purchaser chose not to return the package and instead continued to use the software after seeing the license, they could be considered to have accepted the contract.

That said, the purchaser in that case did check an “I agree” box in the installer rather than try to bypass it – but at least based on the Wikipedia article, it sounds like that fact wasn’t as crucial to the outcome as one might expect.