Mainly because they way they do it is cruel and unfair and usually hits those that need the help by starting off with the objective to actively refuse and punish not to actually assess and help?
Also possibly because benefit fraud is tiny compared to mistakes (usually in the governments favour) in decisions and the amount not claimed that people are eligible for?
In the case of covid the government ignored even the most basic of any fraud measures, they didn't require companies to have filed taxes, or to have been registered prior to March 2020, and didn't bother doing things like checking that the people applying for the money were actually working for or owned the companies, or that the cash was going into accounts that the government knew were linked to the companies that were supposedly applying for the loans.
This is stuff that could have been checked in a couple of minutes per application by simply looking at the governments own records at HMRC and companies house before fraudsters got tens/hundreds of thousands at a time.
And I won't mention the PPE fraud with the VIP fraud lane that meant if you were a mate of a cabinet minister and had just started a business with £100 in the bank you could get a contract to supply a couple of hundred of thousands of bin liners as PPE faster than any approved supplier who had actual PPE in stock could get an answer on the phone.