They don't. We just haven't seen many larger bound states yet. Note that a tetraquark is different from two mesons, and a pentaquark is different from a baryon

They don't. We just haven't seen many larger bound states yet. Note that a tetraquark is different from two mesons, and a pentaquark is different from a baryon+meson. As you increase the number of fundamental particles in a particle, modelling the exact mechanism of the bound state becomes harder. So for larger particles, we aren't even sure they should exist. IIRC pentaquarks were something that theoretically were expected to exist, but were overall experimentally elusive.