The fact that Phazotron has demonstrated an AESA radar (even if it was only 600 T/R modules) proves that they can produce the modules. It is expensive, I know.
They have already moved on from that radar and improved the electronics so the antenna can be moved further back in the nose making room for a larger array with just over 1,000 elements.
Of course they could produce them, but it doesn't make sense to make them in large numbers till the technology is mature.
Think of it in terms of light bulbs... 20 years ago LED lights were simply not an option to replace standard house light bulbs because in terms of technology they weren't developed to a point where they could offer any advantages... they would generate less heat and use less power but would be too dim to replace a standard light bulb effectively.
With investment and development and experience new LEDs are much brighter, don't generate much heat (wasted byproduct) and while they use a higher voltage they use much less current and therefore are cheaper to operate. They often also have very long life spans.
AESA in Russia is at the point where performance would be slightly better than PESA for the same size antenna, but the electronic processing needed and the radar elements themselves means an AESA radar with few advantages over a PESA would cost thousands of time more because it contains a thousand times more transmit and receive modules.
Right now it just makes sense to keep using mature PESA designs which are actually very good already and wait for AESA technology to get smaller and cheaper.
The new Gorshkov Frigate has four enormous radar arrays on its mast for the new Poliment radar system... a similar system for the S-400 and likely another system for S-500 will also be in development, and ground based and sea based platforms will also have AESA radars in development... they will unify the transmit receive module design, so instead of making 1,000 per aircraft they will likely be making 10,000 to 15,000 per radar array for Ships and large land based antenna arrays... such mass production will lead to design and manufacturing improvements and size and weight and cost reductions... like I said get the aircraft into service and then let the AESA technology mature before you start introducing it widely into service... there is no huge rush at the moment to warrant the expense of rushing them into service now.
The AESA radar in the Mig-35 is not made by Mig, it is made by NIIP and they have been working on AESAs for some time... getting them into service on Mig-35s is a good way to get them into service and into production and to work out the kinks and to learn about what they can or can't do very well. LPI and ECCM modes need to be learned...