Because they were determined to get some political dirt, even if they had to break the law to do it. Sound familiar lol?
FISA is a joke. Their standards are so low it's practically pointless. Yet even with the low standards FISA turned down several surveillance requests---until they paid Steele for his fake Dossier. It's clear now the Dossier was intended to provide a justification for surveillance before the FISA court.
I don't think it was intended to be taken seriously, beyond that.
And I think these people knew that Russian collusion was fake news. I think they have been fishing from Day One. Notice how quickly Mueller turned from collusion to a something/anything investigation on Trump.
This is some serious high level corruption and it's more extensive than Watergate.