I also wondered why there's no freeway from the San Jose area to the I-5.
There are some plans, but there's either currently no funding or no will to make the upgrades. VTA has obtained buy-off from San Benito County to upgrade CA 152 into a four- to six-lane freeway, along a partially new alignment -- first along the northerly couple of miles of CA 25, then a new alignment through northern San Benito County back to the existing CA 152 near Lovers Lane, and finally along the existing CA 152 to CA 156. (This buy-off took over 20 years to obtain).
CA 152 is currently a four-lane expressway between CA 156 and Los Banos (a.k.a Pacheco Pass), with a handful of existing interchanges -- more would be required to convert to a freeway, along with wider shoulders through the more mountainous parts of the pass. To the best of my knowledge, no one is looking into such an upgrade right now.
A freeway bypass of Los Banos is proposed, but only funded for about half of its length (between roughly I-5 and CA 165).
The rest of CA 152 is, like Pacheco Pass, a four-lane expressway, again with a handful of existing interchanges. More interchanges would be required to convert it to a freeway, something no one is pursuing right now.
That's the thing we have to remember with a lot of highway setups is that they were designed for the population patterns of that era.
That, and commute trends that could never have been dreamed of in the 60's, such as thousands of people living in the Central Valley and commuting into the Bay Area. Two of my co-workers do this every day.