For several reasons:
Prejudice
This is the big one. Wizards often tended to look down on Muggles (and indeed other magical creatures). This reached its height, of course, in the pureblood supremacy and Muggle genocide and subjugation often advocated by Dark Wizards, such as Grindelwald, Voldemort, and indeed Dumbledore in his youth.
However, it is present in many more subtle forms throughout the Wizarding world. Many sympathetic characters, such as Horace Slughorn, while quite opposed to the murder of Muggles or Muggle-borns, and being friendly to Muggle-borns or even Muggles in general, still held prejudicial attitudes toward anything that hinted of Muggle origin.
“Your mother was Muggle-born, of course. Couldn’t believe it when I
found out. Thought she must have been pure-blood, she was so good.”
“One of my best friends is Muggle-born,” said Harry, “and she’s the
best in our year.”
“Funny how that sometimes happens, isn’t it?” said Slughorn.
—Harry Potter and the Half-Blood Prince
We see here that Slughorn displays the belief, widespread in the Wizarding World and entirely nonfactual, that Muggle-borns and blood traitors are of inferior magical power to wizards of supposedly older stock.
Even Arthur Weasley, whose son was a self-professed blood traitor and who was dedicated to eliminating anti-Muggle sentiment in all its forms, nonetheless held a mildly condescending view of non-magical folk, which perhaps serves as testament to the enduring strength of such stereotypes in Wizarding Britain.
One might believe that Muggle-borns and half-bloods would tend less toward this, and indeed they seem to be less prejudiced toward other Muggle-borns and half-bloods than many of the pureblood families are.
But prejudice against Muggles is not just about origin: it is about power.
Regardless of a wizard's origin, they possess far more personal power than virtually an non-magical person.
Wizards can accomplish feats that Muggles simply cannot.
Many Muggle-born wizards were likely subject to bullying from their neighbors or peers before they went off to school, since odd events tend to happen around magically inclined children, which might cause people to fear them, or more likely scorn them as "odd". They therefore are transitioning from an uncomfortable environment to one that is far more accepting, where the apparent difference is between wizards and Muggles -- at a young age no less!
The society that they enter already has strong prejudices against Muggles. It is easy for Muggle-borns or half-bloods to adopt them to fit in, or at least to pretend to scorn anything Muggle, even if they do not truly feel them.
Magic substitutes for technology
Many Muggle inventions simply are not useful to wizards. This also can cause them to dismiss the ones that could be, which overlaps somewhat with the previous section. It might also give them less reason to interact with Muggle systems in general.
Transportation of any kind is virtually useless to adult wizards, and almost useless to child wizards. The one exception is when blending in with non-magical individuals is a high priority. Apparition can instantaneously transport someone hundreds of miles, as can the Floo Network or Portkeys. Brooms can travel at more than 100 miles per hour.
THE FIREBOLT HAS AN ACCELERATION OF 150 MILES AN HOUR IN TEN SECONDS
AND INCORPORATES AN UNBREAKABLE BRAKING CHARM. PRICE ON REQUEST
—Harry Potter and the Prisoner of Azkaban
With all this, what need do wizards have for Muggle transportation.
Similarly, wizard healthcare is in most ways superior to the Muggle variety, with wizards routinely living past 100. Something like missing bones, which which a Muggle would never recover from, can be healed in a single night. Broken noses are fixed with the flick of a wand. Huge gashes can be healed fairly quickly.
There are many such examples.
Further, magic interferes with technology:
"All those substitutes for magic Muggles use - electricity, computers,
and radar, and all those things - they all go haywire around Hogwarts, there's too much
magic in the air."
—Harry Potter and the Goblet of Fire
Combine all of this and we see that wizards actually lack many reasons to interact with Muggles.
Enforced isolation
What many people don't realize is that Wizarding isolation is largely by necessity. In the Harry Potter universe, people did not often react well to the presence of magical folk in their communities. While adult witches and wizards were fairly safe from attacks, their children and property were often vulnerable, and they suffered a great deal of persecution. This led, of course, to the creation of laws such as the International Statute of Secrecy.
The point is, even the least biased Muggle-born wizard or witch could reasonably feel that they would put themselves in danger by interacting too much with the Muggle world, either by losing a friend who got too close and discovered their secret, or risking the exposure of wizardkind, the prohibition of which has been drummed into their heads since the first year of Hogwarts. They might also worry about physical harm to themselves or their children.
Finally, they might even worry about what the Ministry might do if their friend didn't take it well. Memory Charms are not morally unproblematic!
All of this combines to create an environment in which witches and wizards don't need to interact with Muggles, disdain doing so, and genuinely risk a great deal in doing so -- most of which can apply to half-bloods or Muggle-borns just as easily as to purebloods.
