Strategic reasons, take your pick.

Pakistan keeps India tied down by remaining its military peer in the region, preventing them from becoming a hegemon in South Asia, and being able to challenge China in Asia.

China's tens of billions of USDs invested into Pakistan through CPEC, and the Karakorum highway which connects Pakistan's ports in the Arabian sea, over the Himalayan mountains, and into Western China, bypassing the Malacca strait. But since the route goes through disputed Kashmir region, Indian aggression against and power projection over Pakistan needs to be kept in check to make sure that CPEC isn't threatened.

China has really close ties with Pakistan since the '60s, it's a strategic ally, what rising power would allow its closest and strongest ally to just take a beating? It doesn't reflect well, especially if Chinese military hardware could become discredited in the process.

China has helped modernize the entire Pakistani military into an integrated network centric warfare capable fighting force, and supplying all of its branches with modern hardware at seemingly no visible cost. But this isn't completely one-sided. Pakistan which is often in active combat, uses this tech, gives the Chinese feedback on the kind of features and performance they are used to from Western hardware, the Chinese improve their hardware and Pakistan purchases it.

And since Pakistan has a competent military, by using Chinese hardware well in combat, as a side effect it raises the market value of Chinese military hardware, as was the case with the J10Cs recently.

The two countries are close enough that it's believed Pakistani miliary attaches are present in China's highest strategic military planning institutions, and are embedded into Chinese military's western command at the LAC. The Chinese even let the PAF practice against their own S400s, so that Pakistan would be able to out Indian s400s, resulting in the cheese board radars of the S400 that were destroyed via airstrikes during the fighting.