[badmintonbaseba]

The cows graze in parks shared with people. You don't want to fence around the people, just the cows. This is an example of a field that the cows visit regularly:

https://maps.app.goo.gl/iYbCeJu8fvrmuAXz9

[strken]

I suppose they didn't feel a stile, gate, or cattle grid would be aesthetic enough.

[badmintonbaseba]

There are already barriers like that at the edges of the common:

https://maps.app.goo.gl/1anrXAPRE3Dp3ZFo9

https://maps.app.goo.gl/YTfAYamG55SftWay6

https://maps.app.goo.gl/AWKxr76HUzgvSqBG7

People have free access to the river and the field, any physical barrier would limit that.