At least for Australia and New Zealand, who are probably the most biosec paranoid countries, there are a large number of reasons. mostly do to with introduction

At least for Australia and New Zealand, who are probably the most biosec paranoid countries, there are a large number of reasons. mostly do to with introduction of pests and other potentially ecologically damaging effects, see introduction of cane toads. Those pests can be anything from viruses to mites (see current bee crises) through to domestic animals. Part of this paranoia comes from the evolutional isolation, and part of it comes from the fact that they are islands, and so it is feasible to implement measures to block the importation of foreign materials. Food, unless it has been thoroughly cooked or cured, is a risk and the diversity of stuff people bring back is enormous. Obviously Australia and New Zealand import food, but that requires licences and is regulated and inspected. I would have thought that land border inspections would be less paranoid about food provided that the neighbouring country has similar regulations. But I guess that better describes e.g. mainland EU borders or the US-Canada border rather than the US-Mexico border.