Firstly because "lab studies" CANNOT prove a vaccine is effective. They can only suggest it is.
The likelihood IS that they WILL have SOME effect, but we don't know how much - it's unlikely to be as good as the %'s reported currently, but quite how much less is a complete unknown.
See for example the AZ vaccine against the SA variant - efficacy against stopping symptomatic illness fell from 65% to almost nothing, despite the same type of lab studies showing it was "effective".
We don't yet know the efficacy against severe illness with any of these variants, it's not likely to have been affected as much, but it will have fallen. So for healthy vaccinated individuals, these variants present only a tiny increase in risk to themselves.....
But the most important factor with the Indian variant is transmittability. A small fall in efficacy could strip away almost all of the vaccines' ability to limit onward transmission (vs the ~80% reduction we've seen after one dose with the Kent variant).
We've been able to come out of restrictions so far this year because the vaccination program has resulted in R being reduced, more than balancing the increase in R from fewer restrictions. Take away THAT vaccine effect AND introduce a more infectious variant, and R will suddenly be quite high and infections will quickly soar.