Ah, a fair question.
It’s because Snape did something far cleverer than merely “shielding his mind” from Lord Voldemort.
As one can make out from their interactions, Snape seemed to be the only Death Eater whom Voldemort ever had any modicum of respect for. You don’t merit Lord Voldemort’s esteem by being incompetent or stupid. Snape clearly earned his spot as Voldemort’s most revered servant by proving himself and being the asset that he was.
See, Snape never lied to Voldemort. Snape knew that Voldemort’s skill in Legillemency would immediately alert him to duplicity, so instead, Snape only told Voldemort the truth.
When Voldemort first returned, Snape justified his initial absconding from the Death Eaters by saying that he thought Voldemort had been vanquished in Godric’s Hollow. In The Prince’s Tale, we learn that this is actually true. Snape thought Voldemort had gone, and it was only Dumbledore who insisted that he would one day return. Later, he told Bellatrix that Voldemort forgave him for impeding him in his plot to purloin the Philosopher’s Stone because he did not know Voldemort was the mastermind behind the enterprise, and only thought that “unworthy” Professor Quirrell (whom Snape hated for getting the Defense Against the Dark Arts teacher position) was trying to take the Stone for himself. Again, this turns out to be completely true.
Snape then spent 2 years “spying” on Dumbledore, and relayed what little he knew of the Headmaster’s activities to Voldemort. Dumbledore chose to keep Snape in the dark on most of his plots, which was actually (justifiably I might add) a source of great frustration for Snape. Then he killed Dumbledore just as Voldemort wanted.
Before the Battle of the Seven Potters, Snape gave Voldemort the correct date of Harry’s departure from Little Whinging. He correctly pointed out that Yaxley had been given a false trail, and truthfully divulged that the Order of the Phoenix distrusted the Ministry and the Auror office and wanted nothing to do with the institution. Re-read the scene from The Dark Lord Ascending in the books and pay close attention to the description of Voldemort’s body language. He hangs on to every word Snape says with great interest, and invites Snape to sit by his side. Meanwhile he ignores and then silences Yaxley (whom he shunts beside Dolohov), and expresses contempt for the Malfoys and Bellatrix by humiliating them. But Snape he holds in far higher regard, arguably valuing him more than anyone save for his beloved Nagini.
Voldemort was quite correct in recognizing Snape as an impressively talented and exceptionally intelligent wizard. He just never calculated that Snape’s cunning could be used against him. Even if Voldemort had decided to raid Snape’s mind, he would’ve found little of interest. Snape’s love for Lily Potter was already known to Voldemort (and he foolishly underestimated it, just as he did with Lily’s love for Harry), and the fact that he spent a great deal of time scheming with Dumbledore would not have perturbed Voldemort, but would’ve pleased him. That was literally the job that Snape was given; to earn Dumbledore’s trust and spy on him, and then relay his plans to Voldemort.
Snape’s deception of Voldemort is honestly one of the most underrated feats in the entire series, not necessarily because it was magically impressive (although it was), but because it was carried out so cleverly. As was made clear by his potion riddle all the way back in Philosopher’s Stone, Snape’s greatest talent was his cleverness, which so many wizards seem to lack. Hermione was quite right in recognizing Snape’s genius.
Snape fooled Voldemort with the truth, not with lies. That’s how he got away with it for so long.
**Edit**: As usual, I’m not sure why the Quora algorithm has favoured this answer, but thank you to anyone who followed me because of it. I greatly appreciate the support. :D
If you’re interested in reading an in depth comparison between Alan Rickman’s and Ralph Fiennes’ performances as Snape and Voldemort respectively, check out this answer: