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That's just optics. There's no particular reason aside from not wanting to pay for the launch costs of having a huge antenna or antenna array at geosynch elevation. Even NASA is designing monstrous arrays for deployment in space so they can spot earthlike planets lightyears away, although the new mandate may have changed the schedule. But apertures can be synthesized. The actual size of the antenna isn't such a major factor so long as the antenna can pick up signal.

That's just optics. There's no particular reason aside from not wanting ...

Other than not being practical for a number of reasons - I'm not going to delve into fanciful 'designs' that again, aren't in the least EVER going to be practical ... synthetic aperature RADARS work well when you can saturate a target with pulses, taking a trade-off in other parameters; parameters you don't have 'to trade off' from a platform in geosynchonous orbit ...

The actual size of the antenna isn't such a major factor so long as the antenna can pick up signal.

I sense a total lack of appreciation for this 'field'; received signal strength (and transmit ERP) for *any* system is directly proportional to antenna gain - a factor which is directly controlled by 'capture area' which relates to an antenna's size ... the ONLY way to make up for a signal which has 'sunk' below the noise floor (intrinsic noise in the first active element e.g. as a front end preamplifier) is to increase antenna gain via an increase in it's size there by affecting the S/N (Signal to Noise) ratio ... even so-called Pulse Compression 'chirped' systems which can function with a N'S ratio require a workable N/S ratio prior to 'pulse compression' (in the receiver) and the resultant S/N improvement!

There is no cheating nature and the 'fact' of thermally induced/created 'noise' in all active devices (and even passive devices!) as used in any RF/RADAR/Radio system!