Why can't we find a broad spectrum antiviral drug to viruses like antibiotics to bacteria?
One of the main issues with nucleotide analogs is that they can also affect the host replication and transcription (though they have a higher affinity to the viral polymerase), and can therefore have adverse side effects. On the other hand, the broad spectrum anti-bacterial antibiotics specifically affect prokaryotic cellular machinery.

(Note that some antibiotics like streptomycin do affect the host mitochondria and thereby cause toxicity. Ototoxicity and nephrotoxicity are known side effects of streptomycin.)

Nucleotide analogs are actually used as broad spectrum anti-virals: For example, Lamivudine is used for both HBV and HIV, which are different types of viruses.