Not just the tech giants
Every time you use a credit or debit card in a supermarket, the transaction (including the items purchased) is logged by the store in a permanent customer profile. They use this both in aggregate to decide on stocking and more specifically to offer "incentives". The only real difference between this and the actions of the behemoths is that the latter can cross vendor boundaries, but the principle is the same. If you use cash you can't be profiled as each transaction is independent and anonymous. Long live cash.