It's not just the tax. Yes there is customs duty on foreign
beers but local beers are just keeping to similar pricing.
You have to bear in mind, Singapore economically is comparable
to a city like Hong Kong, Shanghai, London, Paris etc. In any of
those cities you'll pay a steep price for a quality beer in a nice
bar.
If you want cheap beer, drink in a coffee shop. As the saying
goes 'you get what you pay for'.