Because impedance is just one part of what determines a headphone's amplifier needs.

It's not about impedance, it's about efficiency (or a related metric of sensitivity). The efficiency of the Sundara is lower relative to many other headphones of the same impedance, so it needs more power to get to a given volume level.

It's about drive, it's about power.

The Sundaras have an efficiency of 94 dB/mW, which is on the low side.

For example the AKG K371 (headphone) have an efficiency of 114 dB/mw.

So with the same amount of energy the K371 are four times as loud as the Sundaras (each 10 dB is a doubling of volume).

This also means that to reach 114 dB the Sundara requires >70 times the energy (> 70 mW) required for the K371 (a 3 dB increase in volume requires double the energy).

In practical terms this means while your phone you can drive the K371 for pretty much all music (even very dynamic or quietly recorded one) and in allmost all situations, you will not get usable volume with the Sundara for many use cases. The same may well go for your laptop or tablet headphone out. On these sources dynamically limited pop productions listened to in a quiet environment at a sensible volume may well be fine while with dynamic orchestral recordings listened to with a bit of background noise you will not be able to get things loud enough that you can clearly hear the quieter passages. In this case you need an amp which can provide the additional power.

When considering efficiency and power requirements you shoud also consider EQ. In the case of the Sundara oratory1990's settings for the Sundara have a 7.5 dB reduction in volume (preamp setting to avoid digital distortion due to the 7.5 dB lift in the bass). If you use that EQ (and do not use any EQ with a negative preamp setting for the K371) we are now looking at 27.5 db difference in efficiency. So you need >500 times the energy for the Sundara. For 114 dB, which you may want to reach very temporarily e.g. during recordings of large orchestras, you now need >500 mW, which is more than a lot of dedicated headphone amps deliver.

Cos Impedance is only one aspect of what makes a headphone easy to drive or not... A headphone being easy to drive is a function of its impedance and it's sensitivity.

Even though the Sundara has an impedance of 36 Ohms... It's sensitivity of 90.46 dB/mW makes it actually hard to drive since you'd need more power expended to get it going

I have a feeling like when some people say "headphones are hard to drive", they mean "they do not sound beefy enough" instead of "they are quieter than the other headphones".

And then you add output impedance to the mix.

Its not hard to drive....if you think about it 94dB/mW means for you can get to 94dB with 1 mW of power....an iphone dongle can put in 33 mW into 32 ohm. at 85dB is avg listening level you want to listen at 8 hours before hearing loss occurs. Doing the math an iphone dongle can get you to 109dB with 32 mW, so 109-85 = 24 dB of headroom. All of this is very conservative math thats probably way too much headroom

As I said, there are use cases where you are OK with a low-powered source - and there are ones where you aren't.

For example:

I have a hearing imbalance so I reduce the right channel by 6 dB (quadruples power requirements) and I would run this using oratory1990's EQ settings, which in turn decreases volume by 7 dB. So that is a reduction in max volume to 96 dB.

With music with limited dynamic range 96 dB peak loudness is fine. So when listening to most pop or rock the Apple dongle would still work. But I also like to listen to recordings of symphonies at a level comparable to that in a concert hall. This means that momentary peaks in the music can go up to ~115 dB (average listening volume is much lower). Clearly the Apple dongle cannot provide the power required for this.