Labour have said many times that to fund the country long term the economy needs to grow. Something which this government has failed to do to any meaningful level in 14 odd years. GDP per capita is pathetic, general productivity is pathetic, living standards - falling and in some cases lower than Eastern European countries who everyone use to look at and point and laugh. The far left won't like the arguement about the economy but it's entirely true.
If you get people back to work by helping to fix the NHS, skills shortages, inward investment have a proper industrial / business strategy etc you'll get more tax take.
Just going along with only looking at tax being high or moderate is not going to solve the underlying problems with the country.
If the Tories lower taxes across the board, I would have no issue with Labour saying they would raise it again for high earners. But I can see why they might not want to do that, instead they are concentrating on getting more people into work.