Put the numbers side by side and the vulnerability speaks for itself. The satellite signal travels ~20,200 km and arrives at about −160 dBW. A jammer is much closer and only has to transmit one way — the inverse-square law (Inverse square law) works entirely in its favour.
A calculation by way of illustration: a jammer of just 1 watt can blind a simple C/A receiver over a radius of tens of kilometres; a few hundred watts cover an entire battle area. Compare that with the tens of kilowatts of peak power a radar needs (Peak power and average power of the pulse radar) and it becomes painfully clear: disrupting GNSS is dirt cheap, protecting GNSS is expensive.
A receiver that is already following the signal ("tracking") tolerates more jamming than a receiver that still has to find it ("acquisition"). That is why start-up — a cold receiver looking for its first fix — is the most vulnerable moment. A weapon or vehicle that enters the area of operations with a track already running and supported by INS (Protection and anti-jam) is much harder to deny.