One of the tenants of mainstream liberalism is that it tends to deal with issues on a case by case basis depending on the specifics of each case, so this is why there tend to be so few great broad based liberal philosophers after the initial philosophers who provided the initial outline of liberalism.
Isaiah Berlin who lived from 1909-1997 has been described as 'the last great liberal philosopher.' His biggest contribution was to greatly expand upon Emanuel Kant's concept of humans as 'crooked timber.'
https://en.wikipedia.org/wiki/Isaiah_BerlinJane Jacobs who lived from 1916-2006 might be argued as one of the first great more narrow liberal philosophers who moved liberal theory to dealing with practical day to day issues.
https://en.wikipedia.org/wiki/Jane_JacobsOf course, in economics anyway, the first great liberal practical philosopher was Alfred Marshal who lived from 1842-1924.
https://en.wikipedia.org/wiki/Alfred_MarshallI would think by far the best known liberal thinker around right now is Paul Krugman.
However, I think the reason most liberal thinkers aren't well known these days isn't due to any anti-intellectualism but because academic fields are getting more and more technical and narrower. The line is "we know more and more about less and less and eventually we'll know absolutely everything about nothing."
This is adapted from
“Philosophers are people who know less and less about more and more, until they know nothing about everything. Scientists are people who know more and more about less and less, until they know everything about nothing.”
― Konrad Lorenz
I certainly agree that Jordan Peterson knows nothing about everything (or anything.)
Edit to add: I'm sure not all liberals agree that there are no great liberal theory philosophers anymore or that their own views are best applied on a case by case basis.
However, in general I think it's far to say that is the case since the cliche of conservatives think in black and white terms while liberals think in terms of shades of grey seems to be true.
------------------------------------------------------------------------------------
This isn't just in politics either. I don't want to make too much of one data point but I remember when 'Dr' Laura was on the Larry King show during the 2000 election (I have no idea why I was watching this) and he asked her "how can you provide advise to people when you know so little about their situation?"
And she replied "If you live by bedrock principles life simply involves applying those principles no matter the details of the situation."