Because companies merge, rebrand, split off and play all kinds of games to dodge taxes and liabilities. It's not like government departments are going to investigate the history of every single business they might spend some money with, they just buy something and write down what it says on the credit card receipt.
It's not really plausible that Patel, or anyone in the Home Office spent that much on beauty treatments in the last year and decided to stick them all on the departmental credit card in the knowledge that would be publicly accessible and audited. It's equally unlikely that Patel is trying to squirrel money out of the Home Office by using the departmental card to send money to a fake company and doing it so badly that she got busted the first time some dastardly twitter sleuth looked at the publicly available records and caught her red-handed. I'm sure there is all kinds of low level corruption going on, there is wherever there's money, but people have got to do better than this because if there is a story there it's been undermined already by nonsense about her spending 70 grand on getting her brows done. And I bet she'd far rather people were talking about that then about whether she gave the nod for the police to kick peaceful protester's heads in last night.